What can I say other than that I agree with your thinking:
the grammatical distinction between prepositions governing
the acc. and the dative would be carried over as a feature
borrowed from the Germanic substrate. Granted it would no
longer matter which prep governed which case in Latin, but
rather whether they indicated motion or location, plus
some cases of perhaps mistaken identification between
Latin and Germanic prepositions. In Old High germanican
you should also get a number of preps governing the
genitive, of course.

(Come to think of it: Pete do you have an Old/Middle High
German grammar around? I think we should get one...)

On 30.3.2008 Peter Collier wrote:
 > The language is VL based. Ablatve forms do merge with the
 > accusative, and so logic (and OTL fact) moves all the
 > prepositions over to governing the accusative. BUT, and
 > this is where the grain of doubt enters my mind:
 >
 > ~ The Romance languages lose case disitnction generally,
 > and quite early on - my Romconlang does not.
 >
 > ~ In (very) Old French at least there is only a
 > Nom/Oblique dsitinction (I'm not familiar enough with the
 > history of the other natangs to cite here), so the
 > prepositions could be said to govern the oblique case,
 > rather than the acccusative. Either way, the preopositions
 > do not govern *nominatives* - but IMC the accusative has
 > merged with the nominative and there is only a Nom/Gen/Dat
 > distinction.
 >
 > ~ The pre-Roman substrate in the region uses the Dative
 > also for indirect objects, and in situations where
 > (Classical) Latin uses/used the Ablative.
 >
 > So I have poor old Octavio who is confused, because his
 > teacher is telling him his writing is inaccurate because
 > such and such a preposition governs some 'ablative' case
 > he has no practical concept of (other than it sometimes
 > seems a bit like the dative, which he uses a lot), whereas
 > in the same circumstance that Gaulish guy on the market
 > stall is using what sounds to Octavio like the nominative,
 > but his Chattian grandfather, who quite frankly doesn't
 > speak Roman so well anyway (and whom they say has never
 > been right in the head since one of Varus' lot nearly did
 > for him 50 years back at Teutoberg) keeps cuffing him
 > around the ear, muttering "it's under*me*, not under *I*,
 > ignorant boy," or something like that.
 >

/Pendecht