GM-Only field on Character Sheet

Is it possible to make a field (like, an input, type="number") only editable by GMs on custom character sheets? (Meaning, it shows as read-only to players)

All sheet code is shared among the player's and gm alike.  Maybe an API script (gm-only) could be written to edit/update an otherwise read-only field.  Use case; character XP  ;-)

If the use case is indeed character XP I'd like the idea of syncing the characters to a GM controlled sheet. Then when a player updates the "read-only" attribute on his character sheets an API script could check the GM controlled sheet and if a change to the read only field occured it could set them back to the value in the GM sheet. This of course has a couple of problems if other API scripts or calculations in the sheet react to the change of the field (like EasyExperience increasing the level if the threshold to level up is met. In that case the API would change the value of the xp attribute back, but the other API script updating the level would still run and not be reversed. Depending on your usecase @Rago maybe we can find a solution for what you're trying to achieve but the short answer is no, no built-in functionality exists that does what you want on a general scale.

I have 2 use-cases: 1. Creating character sheets for sessions aimed to players completely new to RPG (and roll20), avoiding so they change values they should not change outside very rare context (like character's base abilities) 2. Creating a new board game where most characters data are fixed; for example, the stats of characters of Hero Quest board game (they have a fixed base, different for each character).

A couple API scripts that may work for you;  [Script] PublicSheet — public read-only versions of character sheets and maybe  [Script] SheetChangeTracker

Mmmmmmm, Jakob's script seems very promissing; I'm gonna try it. Thanks!