Well you will have "|transcript=" in most of the possible options anyway. The question is how do you want editors to add transcript links?
- Full link
|transcript = [[{{PAGENAME}}/Transcript|Transcript]]
- Manual trigger
|transcript = on
...or some form of autogeneration, eg. always link and have a red link in every episode page until someone supplies a transcript, or just supply a link for every Legendary Defender episode page and leave the earlier ones alone for the time being.
The "full link" example is the most basic to script into the template, and while the link might look complicated for new editors, it could be added as an example on the template page, and the links won't need to be adjusted once they're in the infoboxes anyway.