The GENERIC kernel with the modified patch (as above) works beautifully, text is restored on leaving X. Thank you again BSDfan666, you really identified exactly the right information!
After that, I compiled a generic kernel with frame-buffer console video enabled plus the above patch. This one continues to exhibit the problem (same as without patch). This case probably won't be so easy, without an existing fix to copy over. Oh well, enough progress for one day.
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Note to moderators: feel free to move the last few posts to a similarly named thread in the NetBSD sub-forum if appropriate.]