If the character is not Rainbow Dash, then it shouldn't be tagged as such. Just tag it with the OC character then. That way people searching for Rainbow Dash won't get OC:KRD instead.
I'll seems to me that a series:* style tag would be more appropriate and helpful for finding and filtering this AU.
Please give me some time to look at what you're doing and see if maybe there isn't a better way to handle what you are trying to do.