I'm afraid what I want to do is a little most specific though. Lets say you already had all traffic on external port 3389 forwarded to port 10000 on host 192.168.2.3. Let's say you also had all traffic on external port 443 forwarded to port 443 on host 192.168.2.3. What I'd want to do at this point is forward all traffic from only host_a on port 443 to port 10000 on host 192.168.2.3.
PF can handle this, since it evaluates NAT rules from the top down, stopping at whichever the first matching rule is (in contrast to rule evaluation, where the last matching rule wins). So there's no chance of a conflict between the "all traffic" rule and the host-specific rule. As with many things PF, you just had to be careful about the order of things. But then if you've ever dealt with any kind of ACL, that's just how they roll.
Anyways, I know this is a kind of specific request, but it's one of two things keeping me from all-out switching to pfsense from openbsd. I'm so used to having ridiculously fine control of PF, it's hard to give up. Even with how sexy pfsense is.