So I've found this patch
It is written around boa-0.94.14rc20 while I'm using boa-0.94.13
I've managed to get it to compile and work by changing the following line:
req->cgi_type = CGI;
to
reg->is_cgi = CGI;
This still isn't perfect though since passing parameters to the cgi page in boa.conf still results in error 403.