At work, we recently upgraded from Oracle 10g to 12c, while (so far) still using a very old version of OpenACS. It's working fine. We did a physical Oracle upgrade using RMAN backups, not export/import, although I expect export would work ok. Oracle has been quite backwards compatible, so any OpenACS upgrade scripts written for 8i will probably still run fine on 12c, but I haven't tried that. We had one (questionably written) query that worked on 10g but failed on 12c (a fairly easy fix), and a few ugly queries that got incredibly slow, presumably from some change between the 10g and 12c query planners. No showstoppers.
So I think your customer's plan will probably work fine, as long as they have the right programmers available to fix the occasional bug.
Wait, or do you mean they want to try to run the latest OpenACS 5.9.1 code, while simply importing the data from their old OpenACS 5.4 system? No, of COURSE that (probably) won't work. If it DID, why would anyone go to all the trouble of writing those upgrade scripts? Keep running the same old OpenACS 5.4 code and everything should be ok. If you want to use that data in a new OpenACS 5.9.1 system, well that's what the upgrade scripts are for.
