I'm not too concerned about getting huge numbers of iterations, just enough for the output to be unpredictable. I'm not sure if my original node made it clear that I'm intending to use multiple PRNGs, for a variety of reasons. I'm also not too worried about sucking up CPU time for the few seconds it will take the user to initialise however many PRNGs are chosen. I would worry about using rand, though, as this would mean that all the PRNGs were seeded in a possibly predictable sequence or set of sequences. I ran into a similar problem about 25-30 years ago, and it left its scars! :-)
Trying ReadKey(-1) gets me about 100K iterations per second without the PRNGs in place, so the solutions that have been suggested here certainly work in that regard. I'm also interested in jaldhar's comments about needing to reset the terminal - it's something else I didn't pick up from the documentation I read. Actually, it looks as though I must have missed some docs altogether. Whoops.
Thanks to all for all the help.
In reply to Re^2: Problem running code until a user hits a key