I looks like the only feasible way to attach wires to our Kapton heaters in vacuum is to solder them on. I can't find any suitable clips/crimps. There are two issues. First, I and Yinzi both found it hard to form a reliable solder joint to the Kapton pads in our initial tests: the surface type and the fact it is thin and holds no heat means that the solder beads onto the wire and doesn't wet to the flat surface of the heater electrical contact. Secondly, solder has rosin and other contaminants that are potentially bad for outgassing and redepositing on optics in vacuum. Furthermore, it is difficult to bake because it has such a low melting point.
I did a quick search for UHV compatible solder, this is what I found
From general reading, apparently lead based solders tend to outgas and should be possibly be ruled out. This has something to do with the vapor pressure of lead. Antimony has similar issues. That said, other sources seem to have no issue with it. The tin-silver solder alloys tend to be a better choice and ones like Sn95Ag5 have melting points of order 230 C so can be baked up to 200 C. Rosen free is better with a separately applied flux that is Isopropyl soluble.
I also wrote to Rich Abbott to see what they did for other LIGO. He said they regularly use Kester 37/63 rosin core solder for a small joints within the LIGO vacuum envelope. This solder is Sn63Pb37 and has a melting point of 183 C and has a 44 Resin RA activated rosin core. The low melting point puts an upper bound on bake temperatures. There is residue from the flux that can be cleaned away with isopropyl. They made a technical note on using these Sn63Pb37 solders with rosin cores in vacuum along with cleaning procedures at LIGO-T1300040. According to Rich there was also an experiment conducted to test its UHV suitability (see LIGO-T1200491) but he didn't have the outcome/results on hand.
So for our heater wire soldering attachment we might be able to get away with using Kester37/63 style solder, if we do a good job cleaning it. Or, we might like to opt for something that is silver/tin based and apply a separate soluble flux at the time of soldering.