Quoting directly from a post by Todd Kopriva, Adobe After Effects employee:
"No. The customization of brushes is rather limited compared to that in Photoshop."
One suggestion is to create your image in Photoshop and use the Write On effect to reveal it.
- The Great Szalam
(The 'Great' stands for 'Not So Great, in fact, Extremely Humble')
No trees were harmed in the creation of this message, but several thousand electrons were mildly inconvenienced.