I also only bottle-condition, there's no kegging in my future (kegs are just too expensive these days and I really prefer my beer in a bottle for portability). Transferring from the bucket to a bottling bucket makes life easy.
The purchase of a cylindro conical (CC) is one of a few things you can do that will dramatically improve your beer. One of the reasons for this is that bottling buckets and priming tablets can be thrown away. Conditioned beer can be prepared in the CC and transferred to keg or bottle without exposure to air or airborne contaminants and it would seem foolish, therefore, to risk these given that you don't have to. An exception would be something like wheat beer which is traditionally conditioned in the bottle by the addition of speise. That is possible to do without the use of bottling buckets. It is also possible to fill conditioned beers into bottles directly from the fermenter and I have done that but it is generally much better to counter pressure fill first into kegs, to allow the beer to 'lager' (even if it's an ale) for some period of time during which it drops really clear and comes to equilibrium with the applied gas and then counter pressure fill conditioned beer into the bottle.
I am assuming whichever conical I buy will be able to withstand enough pressure that I can use CO2 to push the finished beer out of it. So I can drop the yeast and trub, then push the beer.
The ability to hold pressure is an absolute requirement IMO not only because it makes transfer under counter pressure possible but because it permits conditioning in the fermenter.
I might be able to mix in the priming sugar at this point, directly into the conical.
Mixing in the conical would be a bit of a challege given the importance of absoultely uniform mixing of speise or priming solution. You would either have to open the fermenter and insert some sort of mixing device (paddle, Lightnin mixer propeller) down into the cone or use pumps to recirculate from the bottom of the cone to a port nearer the top. I think it would be much better condition in the CC and then CP transfer to kegs first, then to bottles if some are needed. Bottles are just a huge PITA and beer isn't very stable in them because even with a CP filler some air makes its way in. Done right beer which has been CP filled to a keg is good for a year or more and if you need a few bottles for a party you can CP fill them from the keg. If it's a few bottles for a party you don't care about long term stability.
In-line priming injectors seem like they should exist, but my research indicates they're more of a commercial-grade thing, I haven't been able to find anything on the cheap. That'd be pretty perfect, really- something like a Blichmann beer gun, but instead of a gas and beer line, there's a priming solution and beer line. I pull one trigger and it shoots out a measured amount of priming sugar solution followed by the regular beer, getting pushed at +3psi or whatever out of the conical.
If you insist on bottle conditioning you should be able to put something together that would dose accurately measured priming solution into each bottle by simple mod to a Blichman gun. Obtain a syringe, some tubing that fits the syringe's Luer tip (might be a good idea to have it all be Luer Lock but that's probably not strictly necessary) and a tee plus a couple of check valves that fit the tubing. Also obtain a tee that fits the Blichman beer tubing and any adapters necessary to match the side arm of that tee to the syringe tubing. Connect the output of the syringe to one of the straight arms of the syringe tee and an connect a check valve to each of the other arms. Orient the side arm check valve so that liquid cannot flow through it when the plunger is depressed. Connect the other end of this check valve to a length of tubing which runs to a container holding the priming solution. Connect the other check valve to the other arm of the tee and orient it such that liquid does flow when the plunger is depressed. Connect the output of this check valve to the side arm of the other tee. Cut the beer line and install this second tee. In operation you would purge the bottle, draw back the plunger which will fill with priming solution, then pull the beer trigger and while the beer is flowing and then depress the plunger which will force the priming solution into the beer.
Anyway, anyone have any experience with this stuff? Should I just bite the bullet and buy a few used 15 gallon kegs, carbonate there and use a beer gun to bottle?
Yes and yes. I have made lots of beers with CC's using the method described above. I would never think of going back to bottle priming or the use of a bottling bucket. You move up to a CC in order to avoid having to do that. I'm pretty sure that after you have been serving from Sankey kegs for a while you will forget about wanting to bottle (except for parties and you can use a Blichman gun to fill a growler for that).