I had heard that theory on the forumsforjustice page. It does make sense. Disguntingly, it also makes the tip end of the paintbrush a 'trophy'. Conversely, it could make it Exhibit A for child abuse if it was found with blood all over it. Did you hear about that national body of pathologists that looked at all the evidence and said that she was chronically abused? It's on that wiki website.
BTW, the coroner DID find a swath of blood stain on her thigh that had been wiped clean. This could have come from the paintbrush.
Wiki can be edited. The autopsy indicated that she was a healthy child. It did indicate vaginal swelling and trauma, but said nothing about chronic abuse. I have read in other places that that rumor is false.