I was hoping you'd tell me something along the lines of "Because we
already have, you just need to do XYZ".

Unfortunately I also can't afford an extra machine to get the basic
functionality of download queueing.

Well you can only specifiy the total bandwidth for a certain interface,
and all queues which belong to an interface are one-way. You'd have to
have a way to specify download bandwidth on an interface and have a
seperate set of child queues for download. Then you'd have to have a
way of sending incoming and outgoing packets belonging to a certain
state to two queues.

It's doable, but it certainly isn't an internal ALTQ problem.
I read ALTQ and pf were now 'married' (in Theo's words), so I'd think
the problem would require changes to both pf and ALTQ.

I'd have a look at this problem myself, but I'm not good with C. I was
hoping there was some sort of todo list I could petition this to be
added too, because lots of people here seem to agree this is pf's (and
ALTQ's) worst problem.
If there's some sort of bounty system with OpenBSD I'd be happy to set
a bounty so get this done faster.
Kestas