Bruce, I mod a different XenForo forum. What you are asking for is not possible with the native software (that I know of.)
If the XenForo software can't do it then yeah, not possible. Good to know.
If you are ignoring a member's posts you will also be ignoring the threads they started. Why? Because the first post of the thread is ignored by ignoring that user so you cannot see it. I'm not sure how you would follow a thread that you couldn't see the original post in anyway - it would hold no meaning for you if you can't read the OP.
The only thing slightly odd about that (and this may point to a XenForo choice, not yours) is that when an ignored member posts in a thread, then it will (usually) show that person's avatar, and the post (without content) and then offer that you can click a certain set of words if you do want to see the post. Sometimes (for the same reasons as
@BruceK), I will do that because the thread is of interest to me, and perhaps someone whose replies I am interested in has responded and so I want to see the context.
I just mention that because in that case, XenForo does offer the chance to both
a) see that the "ignored" person has posted (you can still choose not to see the contents of that post)
b) click to get like a one-time reveal of what that person said.
So it almost seems like two different philosophies (which maybe is why
@BruceK brought it up).
Maybe it's just an inconsistency on XenForo's part, vs. an actual philosophy. Post in the middle of an existing thread vs. a new thread.
(One more note is that, though I have not figured out the pattern of when/why the difference, but the behavior I described above where you can see that the ignored person made a post in a thread and then choose whether to do a one-time view of it is also not consistent. Sometimes I'll have zero clue that an IP has posted in an ongoing thread unless or until a later poster to that thread quotes the IP. Then I can see the quoted material. I really haven't figured out if there is a pattern to those two different behaviors though. But i have not exactly studied them for clues either.)