Crossloop appears much faster, but involves both parties creating an account and downloading software, not really ideal.
UltraVNC with “SingleClick” seems like a nice free solution, but it is a little more complex to set up (port forwarding, configuring the MD5 key, etc) However, it produces a nice branded “one-shot” server, and deletes itself afterwards, which I found attractive.
We could also set it up in repeater mode, with a server in the DMZ, to avoid direct PFs through the firewall.
Found a package called “4remotesupport” which seems to use UVNCSC as the underlying structure, based in America, with an intermediate server. Better pricing.
Will try that after the weekend.