I feel like there is too much mystery here. I've had status that covers PS on many flights for a couple years, and also have had no trouble capturing the credits. At the risk of spelling it out too much:
1. Use it on PS for someone else or food if that makes sense - for example sometimes I pay for my parents if they're coming to visit.
2. If you truly have no use for it as above, purchase a ticket for a future date. Do not include your AS number if you would get PS free no matter what (75k/100k). Simpler if this isn't on your platinum so that you don't have to figure out which credit you're asking for.
3. In a second transaction, which can be moments later, pay for a premium seat selection. Short flights tend to be small costs. Long flights tend to be more like 100-150 per transaction.
4. Wait at least 4 weeks from the transaction to ensure you don't waste your time by getting told to check back after 4 weeks. Using chat, ask to receive credit for the seat selection fees. Whether or not you still have an existing reservation with AS is immaterial (and certainly don't need to discuss with Amex agent) - Amex agreed to credit for purchase of seat selection fees made with the Platinum.
5. At some point greater than 24 hours after the purchase of the flight, if you do not need to use this ticket, you can cancel and credit all of this to your AS wallet - I'm not lucky enough to be a business flyer and have to buy all my own tickets, thus never have trouble using my wallet funds.

Have done this with Platinum and Aspire x 2 for a few years running without ever having a problem. HUCA very helpful if you ever run into difficulty, it is clearly within the terms of their program to refund seat selection even if AS tends to code these incorrectly.