You're on the right track connecting an HDMI to Component converter from the X1 box to the Slingbox component ports. That said, not all such converters are guaranteed to work with the Slingbox. Here is the only one we know is guaranteed to be compatible with the Slingbox:
As far as the possibility of some HDMI ports being disabled, it's a little more difficult to tell. It could be that those ports won't work when the main one is in use?
Here is what I would try. I would hook up the HDMI converter with the main HDMI port, connect the Slingbox, and see if you get video signal during setup (make sure the cable box is on as well!). If you do, try hooking up the TV to one of the other HDMI ports. If you then do not get signal on your TV, you know it's the HDMI port itself. You then have the option of running a second set of component cables from the Slingbox OUT to the TV.
Hope this helps! Let us know how it turns out!
Thank you Evan!
Quick interim update:
Spent way too much time with Comcast trying to get someone who would turn on one of the inactive HDMI outputs in the X1 box to no avail.
You were right, I tested by hooking up the one active HDMI output (the one that normally goes to the TV) in the X1 to the HDMI-to-Component adapter to test the protocol and the Slingbox worked perfectly. So we have confirmation that what we need is an additional active input.
The third party device you pointed to seems to be exactly what I need; the description in the .pdf is spot on, so I ordered one but it's backordered until the end of the month. This is ideal because it replaces two things, a 2-to-1 HDMI splitter and an adapter-to-Component, both are in the same unit.
Hope it works. I'll provide an update once I get it.