Question:

If a player is not permitted to join a jam because she was not in position at its start, is she then allowed to join a Jammerless Jam that follows?

Answer:

No, such a player was not on the track in the previous jam, and so cannot be a part of the Jammerless Jam that follows.  The following rules logically extend to Blockers:

6.4.5.3 During a Jammerless jam, when one or both Jammers fail to be on the track when the jam starts, both teams will play a Jammer in the new jam.
6.4.5.3.1 No substitutions will take place.  The team(s) must skate short, however they are permitted to field a Jammer from one of the Blockers that played in the stopped jam.