Having a hint that symmetry is on seems useful since as you say - it can be left on by accident.

OTOH, I think colors over the brush outline may be a bit too distracting.

We have something similar for auto-key (red record button in timeline).
Try enable this, then grab the object - notice text in top right hand corner.

Ideally this can be done without making it an option, since users then have to check if they disabled it. But there is some tension between being too distracting and having it easily ignored.

This is mainly a design issue - what do others think of think of showing text on top-right that shows symmetry state - which avoids overloading information into the brush display?

But "radial symmetry" or "tiling offset"?

For me at least, it doesn't seem like too much info on the brush since the indicator disappears while sculpting. Yes this is a useful feature while working with asymmetry model. You want the brush to remind you if the symmetry is on or off.
Something similar would be great to have for the lock option. The lock option needs this option so much because it used for in a short amount of time.

PS: I didn't use the patch. How does it apply to master? If it is easy, i will try it and give clear feedback.

@Campbell Barton (campbellbarton) I agree having text in the corner has a good balance in terms of easy to notice vs. too distracting. It's kind of a separate issue but I have always thought the text in the 3d view should be white/light with a black outline or vice versa (which is legible on any background) because of situations like these: (the second image is the auto key you mentioned).

I also think this should include radial, tiling, and especially lock (many times I have forgotten lock was left on). With text in the corner this is how it might look:

Mirror:	X Y Z
Radial:	X Y Z
Tiling:	X Y Z
Lock:	X Y Z

Maybe the X Y and Z could be colorized to be more conspicuous, might sound unnecessary but you can get so focused on a sculpt that the rest of the GUI kind of becomes less noticeable.

Perhaps the brush outline itself could be used; I think most users more often hold down ctrl key to invert the brush (rather than using the button on the brush panel), so there is less need to indicate add/subtract in the paint cursor's color, which could instead be used to indicate less obvious things. Dunno if anyone else agrees with that though.

Also, maybe the axis in the bottom left corner could somehow be used to show this, but again that might not stand out enough.

@Erick Tukuniata (erickblender) to apply patches: https://wiki.blender.org/index.php/Dev:Doc/Tools/Patches

@Bob Smith (uvwxyz) - Mirror, Radial, Tiling could be displayed on a single line. Lock IMHO isn't needed since the reason to show text is to show you geometry not in the view is being edited.

As for text display w/ different background colors - this can be solved separately from this patch (for the common case it works well for now)