Also:
There are a LOT of different minor races (human-ish a good many) that have very different histories. There are a lot of non-humans in the Imperium and beyond.
It seems very hard to come up with a common symbol. Would a symbol that was sensible in Sylea or Vland or Sol have any traction along the borders with the Aslan or the Vargr? Possibly not so much.
My best alternative symbol for a humanocentric view would be a figure placing its hand on a downed companion (helping/healing).
Jeff also wants to include evac ships, various non-directly medical forms of compassion, etc. so that's another issue. Compassion as a concept is hard to embody in a simple icon.
Given the hundreds of alien races covered by the Imperium (maybe thousands), the vast number of different human cultures, and the long messy history of the Imperium, maybe they'd go with something like the EU strategy:
We have too many flags and too many symbols an colours to easily define a common EU flag. Except... we can create a massive rainbow thing without any symbols but including most or all of the colours used by our members.
Maybe some sort of dead simple symbol (for instance, a text character in one of the non-English scripts that happens to translate as Compassion) could be used.
It's either pick something specific to a major culture/political group (Solomani, Vilani, Syleans) that will only be relevant to a portion of the viewers but that can be made familiar (because the Imperial Government uses it ubiquitously (locals can do otherwise) or else try for something so generic that it can be made acceptable for all (like the the messy EU flag).
TomB