This is just a random thought, others can explain why it might or might not work..
The HVAC system uses manifold vacuum to operate the controls. Seems like you could tee into this line near the vacuum resevoir for this system if you moved the one-way valve in this line from it's location on top of the manifold to the resevoir side of your new tee.
Notes based on my '83 with B23, not sure of any changes by '90:
The vacuum resevoir for the HVAC is under the blower motor (easier to get to than that sounds though, remove the passenger center console side panel) and looks like three plastic balls in a row (tennis ball size).
The one-way valve is in a short section of hose attached to a nipple on the passenger side of the top of the intake. This hose transitions to a smaller and more rigid hose for the run to the resevoir, so you would have to consider how to plumb the valve into the hose near the resevoir.
Basically, it is a trade of hose-through-firewall hassle for valve-in-hose plumbing hassle.

Joe

You want to read manifold vacuum, so that means downstream of the throttle plate. I think the vacuum line to the flame trap *might* be ok, but considering its purpose in life the line might be prone to getting crudded up. The line to the fuel pressure regulator is probably OK.

As far as getting thru the firewall, if you're using soft tubing, I think you'd be best off drilling a hole, probably on the passenger side for easier access. Just be sure there's a clear area on the other side (don't want to drill into a wiring harness or an A/C line!) Chris Herbst in another post had a good idea - find a spot that looks good, then dimple it with a hammer and punch, then check the other side to see if the area is clear.

When you run your line thru the firewall, line the hole with a rubber or vinyl grommet - get one at Radio Shack or even a good hardware store. Once the line is in place, seal all around it with RTV to keep the rain and road splash out. Use nylon tie wraps (don't over-cinch them) to secure the line in the engine compartment.