Normally, it seems grammaticalization that leads to split ergativity tends to produce a split where past|perfect|perfective|etc... stuff has an ergative alignment. What would be a reasonable grammaticalization path for the inverse situation to obtain, i.e. nominative-accusative for past|perfect|perfective|etc..., ergative-accusative in non-past|imperfect|imperfective|im-etc situations?
Instead of deriving past/perfect/perfective/etc. ergativity from a nominalised clause where the agent is formally the possessor of the perfective etc. action use a similar path of grammaticalisation for the imperfective side of the divide only. In the same way that a nominal clause like
ReplyDeletehe-GEN eat-RESULT.NZ cake
can turn into a perfective clause with ergative alignment
he-ERG eat-PRF cake
eg. "He ate cake."
a similar nominal clause with an imperfective type nominalisation of the verb like
he-GEN eat-ACTION.NZ cake
might turn into an imperfective clause with ergative alignment
he-ERG eat-IPRF cake
eg. "He's eating cake."
I don't know any cases where the latter has happened and it looks like a less likely process than the former but it might still be possible to derive imperfective ergativity this route while leaving an ancestral accusative pattern intact on perfective clauses.