Well, you didn't state initially that you were looking for a Caighdeán Oifigiúil response.
This misunderstands the status of the CO in the Irish language. The CO is a document drawn up by learners in Dublin. No native speakers speak that type of Irish. Their "rules" are just made up. But let's look at their rule 9.32 in Graiméar Gaeilge na mBráithre Críostaí:
Quote:
Ní úsáidtear an ginideach d'ainmfhocal éiginnte atá ar aon fhoirm le hainm briathartha
agus nach de réir ceann de na cúig dhíochlaonadh a infhilltear é, agus é ag teacht i ndiaidh an
réamhfhocail a, ag, nó i ndiaidh réamhfhocail chomhshuite
I have to put this in good Irish to understand it:
Quote:
Ní húsáidithar an tuiseal giniúnach le hainmfhocal éiginnte atá ar aon dul le hainm bhriathartha
agus nách de réir ceann de sna cúig díochlaontaíbh a hinfhíllthar é, agus é ag teacht i ndiaidh na
réamhfhocal a agus ag, nú i ndiaidh réamhfhocail chómh-shuíte
Well, their own rule says this only applies when following the prepositions "a" and "ag" or compound prepositions.
I would formulate the rule differently: that genitives of verbal nouns are often avoided, and I would scrap all that about "only after a and ag" etc., not because it's wrong as such, but because the rule feels non-comprehensive and unlikely to cover all eventualities.
Peadar Ua Laoghaire has "Coisgidh na fir sin ar an obair atá acu 'á dhéanamh, agus ná curtar suas an chathair sin gan tuille órduighthe uaim-se" in one of his works.
Tuilleadh órduithe.
It is undoubtedly the case that the use of these verbal-noun-style nouns in the modern language has grown a lot compared with the traditional language. I think maybe the ones in -adh and -ú (<-ughadh) would feel the most awkward to use in the genitive. But still, tuilleadh scrúdaithe is correct. See
https://www.gaois.ie/en/corpora/monolin ... Mode=exact for examples from the Corpus of Contemporary Irish (a site worth bookmarking; rather than searching Google, you could search this, as the works in this corpus are all properly produced government documents, laws, books, etc, edited publications).
It also feels weird to say the verbal nouns don't belong to any of the five declensions. This seems to be trying to overcomplicate things, as if you're meant to look down a declension table and try to see where athrú comes in. A more logical approach would be to say: a verbal noun whose genitive is actually a verbal adjective. So the genitive of athrú is athraithe, which is a verbal adjective. Peadar Ua Laoghaire does have "táim chun labhartha leis", and so these "rules" are not solid.