Sounds like a good compromise - your option of a link to contact the Mod team will enable folk in Billy's position or folk serving overseas to still be able to join.
Security's always a nightmare and I daresay the more persistent among them will find ways to bypass whatever measures you put in place, but you can only do so much and this should stop the bulk of them. There are plenty more sites they can target with much less security, so they won't waste time on yours