Well here's my take on it: The MP7, with the stock retracted, is about 13 inches long...slightly LONGER than the MK23, yet much wider. Size wise, it really embodies the "Machine Pistol" idea, unlike the MP5A4 and similar larger SubGuns. The A5 was modeled after the MP5K, which is about the same size...about half an inch longer than the MP7. The A5 is 20 inches long, with the stock 8.5 inch barrel, so without a barrel it's 11.5 inches, if my elementary school arithmatic serves me right. To make it to scale, it would require a 1.5 inch barrel...so obvioulsy that wouldn't work. Another option, however, is to fit it with a mock suppressor, like the Lapco one, to accomodate a longer barrel while maintaining a realistic look...the Mock suppressor would give an extra 8 inches of barrel lenght to the MP7/A5. Since the real Mp7 has either a 20 or 40 round magazine--the latter being exposed and the former being inside the pistol grip--a TacCap would make a great hopper option because it is both low profile and would give you 30-40 rounds, which for a marker that small should be enough to sustain you until you can reload...if not, then you're pretty screwed.
The other idea, as it was hinted at, would be to make a Tac8 body kit. This would be better since it is already a pistol...in this case all that would be required is an extended magazine, a shroud that shouldn't be all too hard to make and some weaver rails...and a stock adapter from the T9 and fit a sliding stock onto it. The T9 barrel, which is compatable with the T8, has a screw on muzzle brake and suppressor, like the MP7, and with the aftermarket barrels that are bound to come out for the T8 and T9, it is safe to say that there is a bright future in the Tiberius marker line's accuracy. Utilizing those parts would make the T8 look pretty mean...actually if you think about it, using the Tac8 would make alot more sense, since you have room to put a longer barrel and rails and such, instead of cramming stuff in with the A5
Just my thoughts on it...if a kit came out, I'd buy it in a heartbeat...after a Tacamo AK kit