I have not used proxypass across different subnets, but there is no reason that it should *not* work. I believe I did setup proxypass once (just for fun) to return an off-site website -- which by extension would imply that subnets are not an obstacle.
The important point is to make sure that whatever you've put into the SME server as the proxypass target is accessible *from the SME server* at the address you have specified. It sounds as though you have the proxypass domain defined on the SME server, and configured to 'resolve locally' - so the SME asks itself "where is xxx.com" and answers itself "right here!".
Login to the SME server using putty / ssh and try to browse to the target url you have specified in proxypass using lynx or elinks, and see what happens.
If the website comes up, proxypass should work. If not, you may have a DNS or routing issue preventing access from the SME server to the intended target host.