While it may be possible with great effort to use the adjustment brush in Lightroom for this task, I think it really is a poor choice. If you have Photoshop or Photoshop Elements, I think you'd get the results you want much faster and with a lot less effort.
I'm getting fairly comfortable with LR (comfortable, but not yet competent)
and wanted to avoid the bother of also learning Photoshop.
So I'm hoping that there is a way to do it within LR.
But Lightroom does not have a way of replacing a background with a solid color. Photoshop does.
If you really really really want to try this in Lightroom ... well, ok, but don't say I didn't warn you ... try the Advanced Healing Brush (non-circular healing) as explained here: What’s New in Lightroom 5.0 Beta? and here Lightroom 5: Advanced Healing Brush - YouTube
THANK YOU - that was a very helpful response.
I guess that's why I've not found the proper commands. (grin)
I'd tried the healing brush, but it didn't look right --
it didn't give smooth transitions among the patches.
So, I guess that I'll have to bite the bullet and learn Photoshop.