In fact not only is it from JMicrom, but my NexStar HX4 is a "JMS 539 PM" chipset and this one is a "JMS 539 B", which suggests that they're quite similar. Nevertheless, it seems that several people with my exact issue have switched over and it's been working well for them.
The modification times on the files from various sketchy firmware sites strongly suggest that the "JMS 539 B" is a significantly more recent chip though, which gives me hope (sort of)?