I was able to host an Actor on my Pod and send a Toot to mastodon and a direct message back to my Pod
Finding the Actor via Webfinger:
Post received on Mastodon:
Sending a message to Solid Inbox:
I had to convert it to turtle manually, but it was received:
I described the details in a (german) blogpost, but google translate might help:
https://datenwissen.de/2020/07/mit-solid-nach-mastodon-posten-geht-das/