walk-on fjord cruise and transfer back to car?

If you look under Timetables on the Norway's best website you will see that all their Flam to Gudvangen electric boats stop by pre -booked request at the intermediate piers, including Aurland.
A straight turn around gives you 30 minutes in Gudvangen.
So yes Aurland to Gudvangen to Aurland can be done 3 x a day
They also run Gudvangen to Flam and Flam to Aurland shuttle buses, so many permutations.
One is to take the first breakfast time bus out of Aurland, the first ferry from Flam to Gudvangen (which will stop at Aurland on request), then get off at Aurland on the return ferry, but many other ways to do it.

Well, I'm not Badger, but here's my assistance.

You are wanting to travel on the Naeyorfjord.

There are 2 ferry companies. The quiet, electric ferry is with Norway's Best.

Here is the link to book the one way 2 hour ferry from Flåm to Gudvangen and return by bus:

Naeyorfjord roundtrip by bus and boat

There is another option for a full day with Norway in a Nutshell. You ferry from Flåm to Gudvangen, take a scenic bus from Gudvangen to Voss via Stalheim, regular train from Voss to Myrdal and then the scenic Flåmsbana train from Myrdal to Flåm.

I doubt we could actually get a ferry to hop on at Aurland, go around
the peninsula and back again and get off

Of course you can! Just note that Aurland is a request stop and the boat will not stop there unless requested. And double check if all departures include Aurland.

As mentioned, you can also make this part of a longer trip.

Thank you all for your most excellent replies. They helped tremendously and I look forward to Aurland!