There's some builds which add this here: https://dcpomatic.com/build/3099-partial-encryption/5a9d0ee924
Let me know if you have any thoughts.

I am in the middle of a trip abroad, from which I will be back after the 9th.
I will try to check as soon as I can.

I tested on a Mac, yesterday, but not with subtitles, I checked out the audio. It worked fine.
I ran ClairMeta on the result, which "complained" about not all assets being encrypted (or not).
The Netflix requirement (I don't know if there are others, but I have come across other DCPs with the subtitles not encrypted) can be found here:
https://partnerhelp.netflixstudios.com/hc/en-us/articles/4417542010387-Digital-Cinema-Package-DCP-Specifications-Requirements
I ran the application without installing on "Applications" and I don't remember if there was any, but maybe a warning that some validating software might object to partial encrypting would be useful.
Clairmeta output on next note. Some irrelevant info was removed from the output.

$ docker run --rm --mount type=bind,source="/Users/ioannis/Documents/PartEncryption/PartEncryption_SHR-1_F-183_EN-is_51_2K_20251102_SMPTE_OV/",target=/dcp_source silvertrak/clairmeta /dcp_source

Status : Success
Path : /dcp_source
Size : 28.20 GiB
Total check : 108
Total time : 79.11 sec

Info(s):

• cpl_0faf11df-bb3c-4822-bc3e-0d2350c0f4e3.xml (PartEncryption_SHR-1_F-183_EN-is_51_2K_20251102_SMPTE_OV)
  . CPL reel attributes shall be coherents across all reels.
  . Encryption should be coherent across all reeels.
  Encrypted is not coherent for all reels
  . Digital Cinema Naming Convention compliance (9.3).
  ContentTitle should have 12 parts to be fully compliant with ISDCF naming convention version 9.6, 9 part(s) found
  Field TerritoryRating not found in ContentTitle
  Field Studio not found in ContentTitle
  Field Facility not found in ContentTitle

[ 1001.77 MiB in 1.95 sec (at 539.25 MBytes/s) ] 100.00% - pcm1_cbf54b0f-241e-4d86-a77a-d29d2118ed93.mxf
DCP - /dcp_source - Check succeeded

I merged this with a hint added.

@carl e8ce097ce705446c27b51199a321a9918deaa0db