Delurking to post some clarifications on the topic, as a speaker of one of the varieties in question.
First of is that in Belgian French the contrast isn't between /ɑ/ and /a/, but /aː/ and /a/.
Second is that it normally doesn't show any traces of the debucalised /s/ in (what are now) open syllables, so none of the contrasts you posit would be expected to exist:
Pâte /paːt/ - Patte /pat/ - Pas /pa/ - Appat /apa/ (/aː/ doesn't exists world finally, but /waː/ does, i'll get back to this)
Être /ɛːtʀ/ - Mettre /mɛtʀ/ - Étais /etɛ/ - Était /etɛ/
but Étaient /etɛː/ (I'll get back to this too)
Île /iːl/ - Fil /fil/ - Dis /di/ - Dit /di/
Côte /koːt/ - Côté /kɔte/
However, there are varieties of French which still distinguish old /Vs#/ sequences from /V#/ ones, but it's mostly found in the speech of some older rural speakers in France, rather than attributable to a big national variety like Belgian or Canadian French.
Lastly, there are distinctions in the verbal paradigm of Belgian French that have disappeared from that of most other dialects. As Sumelic alluded to, /Və/ sequences have yielded /Vː/, mostly but not exclusively word finally (for example I contrast "il lirait" /iliʀɛ/ from "Il lierait" /iliːʀɛ/).
World final vowel length might not always be contrastive in verbs. For example
Essuyer and
Nettoyer have a long vowel throughout their singular indicative and subjunctive present conjugation (/eswiː/ and /netwaː/) but they don't contrast with any verb form /eswi/ and /netwa/, although /eswi/ can be a noun. Here are where they are contrastive:
Person Marking: Generally 3P vs the singular forms, but not only:
- Je vois, tu vois, elle voit /vwa/ vs elles voient /vwaː/
Il ait /e/ vs j'aie, tu aies, ils aient /eː/
Je ris, tu ris, il rit /ʀi/ vs. ils rient /ʀiː/
Je mangeais, tu mangeais, elle mangeait /mɑ̃ʒɛ/ vs. ils mangeaient /mɑ̃ʒɛː/
The suffix -aient is the only instance of open syllable /ɛː/ in the language (everywhere else, it's raised to /eː/) and it's not rare for speakers to just say /ɛ/. Many speakers, me included, also tend to regularise the subjunctive paradigm of avoir by extending the /eː/ to "Il ait".
Mood Marking: Because the Latin Subjunctive Present Active often had /a/ as its theme vowel, it was followed as /ə/ in French and it's not rare the present indicative and the present subjunctive be distinguished by vowel quantity:
- Je vois, tu vois, elle voit /vwa/ vs je voie, tu voies, elle voie /vwaː/
J'ai /e/ vs j'aie /eː/
Je ris, tu ris, il rit /ʀi/ vs. je rie, tu ries, il rie /ʀiː/
Sumelic wrote:Vowel length in French is a complicated topic because the present-day varieties that do have length distinctions don't necessarily have the same distribution of long and short vowels as the commonly-described old-fashioned France French variety that had vowel length
Well, depends on how old-fashioned we're speaking. 17th and 18th grammarians describe similar vowel length phenomenon for both Vs and Və sequences in France French too, and it only disappears from descriptions of the standard during the 19th, appart for ê.