If your shelter uses the foster in and out method, you already have foster PID's "tagged". You should be able to pull a list of foster parents and the pets that they fostered.
Depending on the version of Chameleon you are using, you can also use it to track volunteers. I believe in 44 they came out with a volunteer module. Volunteers can clock in and out which then you can use to create your data. If you don't want to use that (or you have an older version), you could also just use the member window. We use it to track volunteer with Chameleon access, but you could use it however you would like.
We use the outcome subtype and condition to track fosters. For example, your outcome type would be foster with a subtype of medical and a condition of bottle baby. Again if you outcome animals to foster, this would be a way to "tag" fosters.
I hope that helps and isn't super confusing. If you need clarification, please let me know.