I found a working solution though a competing software provider. (Described below.)

Honestly I was surprised and dismayed that TurboTax Desktop currently fails to support section 475 elections in an EFiled return. Such elections are common enough among investors. (And they are explicitly required by the IRS for MTM accounting.) TurboTax seems well designed in many respects so leaving out a basic need for a subset of investors seemed uncharacteristic.

Warning, there is some technospeak in this paragraph: From a programming perspective, supporting a text based election does not specifically require the ability to attach a user-generated PDF document, and supporting the technical problems such a PDF attachment could entail. (Such as EFile attachment size limitations). Making a 475 election possible only requires the ability to accept a "taxpayer note" for addition to the return, meaning accepting a paragraph of plain text. From the text based user input, the TurboTax software could generate a PDF document page containing that text, ensuring that the final EFiled PDF complies with all the underlying technical requirements of the EFile transmission protocols.

For the benefit of other forum readers who run into this problem: I learned that a competing software called TaxAct has the capability to EFile with a section 475 election. TaxAct does have various upsides and downsides from the perspective of a full tax software comparison, but thankfully it solved this particular problem for me. It allowed me to complete and EFile my return.

Here are the TaxAct support links that document this feature. The links describe how to add a section 475 election, or other regulatory notes, or general taxpayer notes to an EFiled tax return.

"Notes and Explanations for E-Filing"
https://www.taxact.com/support/16694/2021/notes-and-explanations-for-e-filing

"Form 1099-B - Traders in Securities"
(Scroll to the bottom of page, where it talks about how to add a 475 election.)
https://www.taxact.com/support/1043/2018/form-1099-b-traders-in-securities