Earlier this summer I emailed these folks
http://www.twowheelmotorsport.com/pro-action-suspension.htm to see if they could service the "progressive 420 shock." At the time they wouldn't commit, they recommended that I bring it in for them to have a look. This shop is located just north of Guelph Ontario and is an hours drive for me.
Today I brought it in, and there was some optimism expressed. He didn't see any evidence of leaking and it may just need to have it serviced and charged. The price quoted was $180 Canadian, parts extra. (Though that included oil and nitrogen) $75 bench fee if it's a loser.
Didn't seem to phase them when I mentioned that progressive in the States no longer stocks parts. I will find out in a week if they can actually do something with it. They will test the spring rate as well. He was keen that all the stats on the shock were stamped on the parts throughout the spring.
I acquired this shock over a year ago at ONSTock If I recall the owner said he had about 40K miles on it and I put on another 15K since installing it.
Prices I've been quoted for the 465 range from $400 to $700. Vicous cycle (twisted throttle quoted $500) Still a chunk of change for a 25 year old bike.