Rob:
Although I have not experienced any problems with archive (pla) files, another approach would be:
1. Do as you did before (save the file as pln then pla)
2. Open the pla file and when you get the "Active Library Usage" dialog box, select the "Extract elements to a folder" option. This will contain the library parts and textures that were saved as part of the archive.
3. Save this library and the original Teamwork file (or pln file if it keeps the fonts correctly) as your archive.
I do not have much experience with TeamWork, but this should work in terms of finding and extracting the project's library parts & textures for an archive.
HTH
David