HMI.rec would need to be linked if you are also using additional Xsubs. The following excerpt is from the readme.txt file that came with HMI.rec:

Using the HMI XSUBs:

Many users will find that they only use the XSUB1, XSUB2, XSUB3,
XSUB4, and XSUB5 with no other XSUBs. These users can load the
HMI.rec file directly. In TISOFT, for example, you would use AUX40
to convert S-records and then AUX43 to load U-memory to accomplish
this.

If you have this PROBLEM:
If, for example, you load HMI.rec into your PLC using TISOFT
(AUX40 and AUX43) and then load your own XSUB, loading your XSUB on
top of the HMI XSUB clobbers it. If on the other hand you first
load your XSUB into your PLC and then load HMI.rec, then you will
write over your XSUB.

Here is a SOLUTION:
The object files are available so that you can use XSUB1, XSUB2,
XSUB3, XSUB4, and XSUB5 while using some XSUBs of your own. You
will need to link in object files. To do this, you will need to
add your XSUB entry points to header.src.

Dave,

I do remember seeing that readme at some point along the way.
What I don't remember seeing in it was any explanation of what data to load into the 18 arguements that XSUB4 requires.
I can kind of infer what is required based on the code that I have that works, but I have no idea how I would have implemented it on my own (luckily, I don't need to, since it does work).
Still, magical black boxes do make me nervous at times :)