I'll try and knock up an example for you, but I'm pretty sure I've faked table-level constraints using a function called by a CHECK constraint in the past.

Could #1 be enforced by a not-nullable FK constraint direct from Services to Resources?

It wouldn't matter too much what you put in there (and you wouldn't be reading the column), but it does force you to put a value in?

There might be a way to use a view with check option to achieve this too.