While LibreOffice and writerperfect (wpd2odt) can both import/convert legacy WordPerfect for DOS files (possibly 4.2) they have problems with French characters. Looking at some files I can see these incorrect characters, but there are others I haven't been able to find (e.g. the c cedilla):
Φ for è
Θ for é
Ω for ê
α for à
⌠ for ô
• for ù
√ for û
(I'll see if I can get approval to attach a sample file, if it's needed.)
You are right. Libwpd contains a WP4.x parser, but it doesn't seem to handle extended characters correctly, and the file libwpd_internal.cpp has a table of WP42 extended characters, but wpd2odt and LibreOffice do not (as you say) seem to use it correctly. The table seems to be correct, but it seems that wpd2odt doesn't correctly recognize WP4.2 files - and that isn't surprising, because the only way to do so would be to scan the file for some specific codes that may not be there at all.
Last edit: Edward Mendelson 2023-12-03
I finally got around to filing a bug with LibreOffice, where it's most likely to get attention:
https://bugs.documentfoundation.org/show_bug.cgi?id=158793
Please look at the package https://ftsoft.com.cz/wp2latex/wp2latex.htm and try your document with WP2LaTeX. I have distilled conversion table for all WP4 characters many years ago. Look at the text file WP4a.enc. I have forged WP4 document with all possible characters and observe how they are imported into more modern WP.
You can re use this table in libwpd.
c cedilla
is not listed here. It could mean, that there were many WP4 coding tables spilled across world.
It is not possible to imagine them.