A scratch build would probably be best. Make your own magazines. I think a JLS F2000 would be a good internal "Inspiration" for a project like this. Just like the AUG, it has a V3 gearbox, bullpup design, etc. The magazine sits in front of the gearbox (Actually, the JLS gearbox cuts into the top of most STANAG mags, so F2000 user have to dremel off part of their magazines to get them to fit & feed properly). My point is, you might not want to confine yourself to a standard design.
Edit: I drew up a concept of this for you in photoshop. Based on WizardOfFlight's blueprints.
I "Measured" the barrel length based on a gearbox (V3 AUG) I had laying in my room. The gearbox was 17.5 cm long. I took the picture of the gearbox in photoshop and duplicated it until I had made them as long as the barrel. 5 gearboxes stacked end to end were approxamately as long as the barrel, so:
17.5 x 5 = 87.5 cm
87.5 cm = 875 mm
PSG-1 barrels are only 590 mm long, so you could end up with a sniper-class weapon (With space for a barrel longer than you can buy!), but keep in mind this is scaled to the Chief. It would be massive. With a barrel that long you have to start worrying about "barrel suck" (The cylinder not being large enough to push out enough air to fully fill the barrel, causing the BB to be "Sucked" back in the barrel. Results in a loss of FPS.).
Two problems I encountered were related to the gearbox - For the gearbox to stay in line with the barrel, the motor would stick out of the stock... Meaning you'd have to skew the stock a little bit. The gearbox would also interfere with the magazine design. If you are aiming for a Halo-Accurate magazine, you would have to cut parts or change it slightly.
Another problem is that the hopup would be too far back to be accessed from the ejection port, so you'd have a makeshift fixed hopup, or an ugly hole in the side of your BR55.
Several people online have built sniper rifles in airsoft by using Gas Blow Back pistols as a base. GBBs expel excess gas each shot, meaning that if you put a longer barrel on one the FPS would rise, and due to barrel length the accuracy would increase. A pistol could be cut down to be smaller than a gearbox. A physical trigger would have to be set up, however... Of course, I probably thought this out further than you did...