There MAY (and this is the key word, "may") be more beasty for ALL girls, as OPTIONAL or even HIDDEN events. I get the hype over this, because I personally would like it, but If I do it, it has to fit into the game and have a plausible scenario for each girl.
And keep in mind that my caution with all of this is not because of the backlash or the "hate" I could recibe, but because I simply don't want to force things to "happen".
But yeah, those events will be ultimately the player's choice. So, there is hope.
I would say that Lucille and Mia's characters do allow plausible beasty scenario's imo.
Mia's trait is curioisty so it would make sense for her to experiment with beasty.
Maybe she reads something on the internet about beasty that peaks her interest.
Lucille is completely oblivious to any sexual activity. She gave the MC a handjob under the pretext of helping the swelling go down by massaging it (HJ) and the MC also had sex with her claiming it was a massage. Wouldn't be that difficult to transfer that to beasty. Lucille sees 1 of the beasty's with an erection, thinks it hit itself on something, wants to help like she did her brother, HJ beasty. After some time she sees the beasty with another erection, damn, she thinks, the massage didn't work, Don't worry though she learned a better massage from her brother. Beasty sex. Would be perfectly in line with her character as she believes she's just helping and not doing anything wrong.
Martha is a bit more tricky though. There's nothing in her character that would leave her open to beasty.
There are still some options though.
Like the Bishop guy said on this thread a dream sequence would allow for Martha beasty scenes.
The only other option would be similar to what you did with the previous Martha beasty scene.
Stuck or something else that leaves her unable to stop the beasty scene from happening.
Could also have the dog sneak into Martha's room at night, since the MC gave her sleeping pills, she wouldn't be aware it was happening.
Normally beasty stuff isn't really my thing but i actually liked the beasty scenes in this game.
Maybe it's the art, maybe it's the immersion into the story.
Just thought i'd give my two cents on the matter. All up to you though.