I'm planning to put a wing on it. I've used the command to add a bodypart to an animal before (most notably in the Velociraptor, when I gave each wing 3 fingers and each finger a talon), but I have no idea on how to select a specific toe on a front foot.
You can select body parts for modifying with specific commands either by TYPE, CATEGORY, or TOKEN
TYPE is the most broad, it can select things like LIMB, STANCE, GRASP, UPPERBODY, ect.
CATEGORY is a little more specific, but still tends to select multiple body parts. Things like FOOT_FRONT, FOOT_REAR, TOE, TENTACLE, PINCER, MOUTH, EAR, ect.
TOKEN will select specific, individual body parts. If you look in the body_default raw file, the token is the entry right after the [BP: ] part of the body part definition. For example, your toes are things like TOE1, TOE2, TOE3.
So, if you wanted to make only one of your velociraptors toes large and have talons, you would use this line in the creature raw.
[RELSIZE:BY_TOKEN:TOE1:35]
This command changes the relative size of the specific body part (TOE1), in comparison to all other body parts. A toe in the 3TOES body plan is usually RELSIZE:7, so this will make the first toe 5x bigger than the other toes.
If you want to add a specific tissue to, say, the first toe, you would need to define a tissue in the creature raw, then you can use this line to select only the first toe for the tissue layer. This example is for adding a tissue called TALON to the front of the toe.
[TISSUE_LAYER:BY_TOKEN:TOE1:TALON:FRONT]