Hello, General. Each token has a GM Notes section that is private, but it is not seen by the Player so that does not fit your request perfectly.
If you associate the Token to a Character, you can do this with the Character Bio tab, or with a Character Sheet, your choice. An upside of this is that Character can be attached to Token
(and you Shift-Click the token to bring up the Character Bio). If using the BIO tab for this, be sure to set the Character Visibility to only the one Player, not to All Players. (The All Players setting allows them to all see the Bio). But even if you have All Players visible, still the Character Sheet tab is only visible to the player who has Edit permission, not view permission, so you can use a Character Sheet tab for this. More-over since you are a PRO subscriber you even have the option of making a Custom Character Sheet, for example you can code your own Character Sheet with a nice big text area labeled "My army list is:".
And another option. You can use Handouts for this, the downside for your request is that Handouts are NOT attached to a token, unlike Characters.
Wiki docs for Characters and for Handouts option,
https://wiki.roll20.net/JournalYour suggestion is good for adding a section that does this in the Token itself. My reply is meant to help you find a way to accomplish what you want in-game for the time being.
