In 2023, when mixer campaigns were at their peak, almost every mixer campaign had additional slots for avatar-only participants.
The campaigns of the manager who opened additional avatar slots are generally not exclusive to the mandatory use of an avatar, so in some cases, it is allowed to use a different avatar in relation to the signature.
RIP mixers

They will be missed.
I think it's kind of grey area for many campaigns that are not explicitly asking participants to wear avatars, but they provide such so it could be assumed/implied that wearing them is a requirement.
We could potentially end up in a weird situation when people could be advertising two competing businesses simultaneously, in the avatar and signature.