hmm... I expect you've tried \& as well then... ?
Other suggestion, (and to be honest this probably won't work but its all I can think of so early in the a.m...) define $USERx$ as the url in resource.cfg, like
check _command check_http!-u $USER10$ -s "test string" -t 60
... I dunno, it's a bit of a silly and non-scalable solution but it might get around your problem...
Else it might be a case of writing and external script and using wget and grepping your test-string...