Well, the status quo depends on an unsupported field - "anchor". As far
as I can tell, the only other option that allows any customisation of
cursor behaviour is to re-implement the entire selection system from the
ground up. Would it be acceptable to detect the Tk version and change
strategy accordingly?

I don't know how far back this would work: I've tested it with Tk 8.5
and 8.4.

I'll ask on comp.lang.tcl and see what they recommend fpr customising
the behaviour of a text widget.

Thanks for looking over this.