Actually there is an upgrade so the TC will fill up in Park. Just had my tranny guy build a customers trans like that. I just found out about it a little while ago.
Yeah it's a revised manual valve and usually comes with a Trans-go kit. It does allow it to be checked in park but I have seen a few that are a little slow filling the converter back up after it drains down over night so if you just start the engine and check it right away it may be off a little. For that reason I still check them in neutral so I know their reading accurately.