thx dutch for your comment, i think i get the point. at least it looks like i am not completeley off road.

the BOINC server should have knowledge of all registered cpids in gridcoin and return only people in team name like '% (Gridcoin)' and with cpid already registered on the gridcoin network. This would keep the total number of miners honest, although it will go up and make mining harder (the price of GRC could benefit it and go up as well). But still the cpids of void miners can be retrieved with other server calls and abused by registering it with a beacon later on in gridcoin, right?

another idea then: assume we go with the second proposal and we add a flag "minesgridcoin" the team founder can set. the upgraded BOINC server allows the flag to be set only if all cpids of the team are already registered in the gridcoin network. this would work at least for small teams. (probably a mess to implement as the BOINC server should not have knowledge of cpids registered in gridcoin, in my opinion)
excuse me just thinking loud, a bad team founder could register all void miners first, set the flag and go away with the additional GRC reward...

best idea so far: in table user of BOINC server add a flag "gridcoinminer". Each user can set this flag individually only after logging in on his/her profile page. After setting the flag, the user will be so keen to advertise his/her cpid on the gridcoin network, the same behaviour as he/she would do when changing the team to 'Gridcoin'.
The new call on BOINC server will return all users who are either in team Gridcoin or who have the "gridcoinminer" flag set. This removes the team requirement and avoids the void miner problem you describe.

the call on the BOINC server will become then:
select u.userstats from user u where u.gridcoinminer=1 or u.team_id=(select t.team_id from team t where t.team_name='Gridcoin'

To me it looks more something that can be solved only involving the BOINC server side, a mod on the gridcoinresearchd alone is not enough... i would give half of the bounty to the BOINC developer who impelments the "gridcoinminer" flag on the user account page and the new BOINC server call.

btw do you already have a sketch idea on how to remove the team requirement?