You wrote....
  AFAIK no ISP uses vintage hardware. So you're
going to be connecting to
 some modern machine. 
 Well, I'm an ISP. Let's see if I can figure out how to get an IP stack on an
 HP2100, SLIP... attach it to a modem bank via multiple baci boards... hack
 up an ethernet board... and voila! Instant vintage dialup server :) 
If that's 'instant', you're a darn sight better hacker than I am....
 Actually, I have thought about moving a machine or two into the datacenter,
 but then it'd be too far to play with, and run my already huge power bills
 through the ceiling! 
When I was at Bristol University, I needed a larger filespace quota on
the departmental VAX cluster. I said to the system manage (OK, rather
more politley than this, but you will get the idea) that he could either
increase said quota, or I'd bring a couple of RL02s and a Q-bus
controller in and stick said controller into one of the microVAXen.
Needless to say he increased the quota...
-tony