You are certainly not the first to some up with this idea, but first to write. I like it. There are some considerations:
What is the use case? What is the point of having hundreds of peers when we don't know what to test? Sure having this system ready in advance will be nice when such a need arises.
Who is going to set it up? If current gridcoin developers (~3) are assigned this task, they would stop developing gridcoin.
I assume we can fake Project RAC with a pre-packaged build?
No need to fake anything. This project could be added to testnet and generate actual rac and mag (not in prod ofc). Or they can be assigned CPID and keys as a part of workunit. You can specify "boinckey" (combination of CPID, secret key and some shady stuff) in your config to run miner separate from boinc.
To generate variability a random amount of coins could be send to each new registered Testnet address.
Good point. Nodes need coins to stake. More thought is needed for this point. How to distribute (pre-loaded private keys?), how to return, what happens when someone steals it.