I've just tried it from X-Terminal into MicroB's URL bar, and the Search box on this site, and they both work with Ctrl-V too The "Paste" button didn't appear in the drop down, though.

I've also tried copying from a page in MicroB use the gestures and Ctrl-C to copy, then Paste in X-Terminal, and that worked too

So it seems the problem is the Copy / Paste buttons do not work correctly in MicroB, but selection, Ctrl-C and Ctrl-V do work.

I've read that the Copy button not working is fixed in unreleased firmware, and I presume that fixes Paste too.

Thanks BrentDC, that makes password entry much easier