Most bruising is from unbalanced feet. The excess hoof material creates pressure, which slightly (or worse) tears the lamina connection, which makes it bleed a bit, which becomes apparent with white feet.
By appearances she has rock hard feet. There is no chipping or flares, her angles are good and improving.
Improving from what?
Feet that are softer/thinner/more flexible will tend to flare more and sooner than feet which are hard. Hard feet will tend to force curves in the coronet band before they flare, assuming things aren’t really bad. How straight, or curved, are the coronets? I do agree that pictures will help a lot. But make sure they are like this - pictures with the lens at anything but perpendicular to what you’re shooting distorts angles and distances 
http://www.all-natural-horse-care.com/good-hoof-photos.html
Mare is landing toe first on the sore hind foot right now.
Sounds like an abscess brewing.
She is on lush pasture 24/7 and has not had any laminitic issues in the past.
How old is she?
Is it possible for horses to get laminitis in the hind feet only one at a time?
It’s possible if it’s a mechanical cause, but really, really unlikely if it’s systemic.
She is not overweight, can feel and just about see ribs, no crest or fat pads. She is fit and healthy but really lacks hind end muscle despite doing hill work, an awesome diet, etc.
Going back to her age - any chance of Cushing’s? How’s the rest of her topline muscling?
100% yes. Know, not think. When feet hurt, they cannot use their body properly. When back feet hurt, they are not going to take the weight back there that’s required to develop sit and push and overally body muscling. If the hind end can’t carry, the front end can’t properly develop. It can IMproperly develop as compensation.
There has also been some mild bullnosing on the hind feet prior to the bruising showing. Could these things be related? I’ve been stumped with her misc. hind end problems for a while now since I pulled shoes a year ago.
Bullnosing is a matter of when, not if, it’s going to cause obvious hoof soreness, and soreness in the hocks, then stifles, then SI, then move forward, it if doesn’t all sort of happen together.
Bullnosing doesn’t happen overnight either. How fast it happens depends on how off the trim is, and for how long, but it is a sign of longer-term poor trimming.