Is there any chance if we can test on the GCP Marketplace if we can offer DoiT's DCI offering as an all upfront payment?

A customer would be very keen to do this and we have the contractual mechanism on this side.

Some Google teams have suggested it wouldn't be possible but Sebastian based on what he could see, seemed to think it would be possible

Per House BP:
Assuming all upfront means annual payment, I see that it is possible to create a private offer for such. However our backend doesn't support annual, so some refinement and development would be needed.

we would still send the excess fee (if any) monthly