Two suggestions ...

Bar's Leak and Subaru Coolant Conditioner

I asked a co-worker who is a master mechanic about the topic. His take was
that the Bar's Leak will gel and plug the leak, he would be nervous about
using it as it *could* plug small channels in the cooling system (heater
cores). The conditioner he felt was a chemical agent that would cause the
rubber gaskets to swell.

Given the choice he would pick the coolant conditioner.

Dave

Jeff sent along this link for the Subaru solution ...

http://www.subaruparts.com/catalog/?section=366

With the following testimonial from a friend...

 Siphon enough coolant out of your EXPANSION tank, not the overflow tank to
make room for the two bottles.
Shake 'em up, pour 'em in. Some on the Samba forums say to run your motor
for a few minutes, then shut it down for a while so it disperses through the
engine and not the rest of the system when the thermostat opens. My weep
literally stopped within a couple of minutes and hasn't come back. Some
folks on the Samba say it has sealed them up for years! I'm hoping it gets
me by til I have time for a head gasket job.