Looks like I can do ipv4 mss limiting with pf. The bloom.host docs say this only applies to port 80, but I'm doing 443 as well to be sure.
scrub out on bridge0 proto tcp from any port {80,443} to any max-mss 1436
I think Bloom's only worried about web DDOS and ignoring other DDOS. Then again, I pay for bandwidth overages, so I guess that's not their problem?