o_O many thanks!
One of the nice things as a homebrewer is that there are no restrictions as to the compositions of one's brews

Rye might be an interesting addition, but I wouldn't call it an Oktoberfest any more... Not that it won't be a tasty brew, it's just not the style any more.
Here is a
great review of recent Oktoberfest brews. The site is in German but one can always run it through a translator...
Oktoberfest is a strong brew. Not really a session beer per-se. It's effectively the step between an Export and a Bock. I grew up with this stuff, so for me a session beer is between 5.0 and 5.6% ABV... guess it's a cultural thing... (or just alcoholism glorified).
If you insist on single step infusion then a longer boil time may be a good idea. Also, if you can convert it to a 2 step infustion mash, ie first thick at about 58C, then at 66C to the final 1:4 water to grist, you should get into a good ball park. I'd adjust the grain bill to be about 1.5-2 EBC darker than what is listed on the link above.
Decoction mashes will be at least 8% more efficient in extraction rates (ie your mash efficiency - not brewhouse efficiency). And since Spelzentrennung is not an option either, the adjustment in final colour should get you in a good enough approximation.
The most important factor is the fermentation schedule though. I always had good results with WLP820 at correct pitch and fermentation temperatures (pitch at 9 Celsius, let rise to 12, then increase to 14 once at 50% attenuation, then hold for a day or two, then slowly down to 3C to rack into lager vessels). If precise oxygenation is not an option, then a starter on a stir plate for an hour or so (until the yeast goes very active), with the original wort from the batch may help things too (your mileage may vary).
Hope this helps.
Enjoy!
