Angus-Y
From what you wrote, I am wondering if you have any version of Premiere Elements at this time. And, what computer operating system is involved?
There are effects in Premiere Elements that can be applied related to Auto Levels, Brightness, Contrast, Shadow/Highlight, Hue, and Saturation, Lighting.You can apply these effects to the whole clip or, for some, keyframe its property so that the effect varies across the duration of the video. But, for selectively correcting a specific area in a frame, you would need to do some exploration and experimentation with the what is. There is an Effects Mask to localize an effect, but how effectively you can deal with blending that into the rest of the frame is questionable and depends on what you are trying to correct. My blog post of Effects Mask is found
http://www.atr935.blogspot.com/2013/07/pe-about-effects-mask.html
and includes how I applied more than one Effect Mask to an image.
There is also a Track Matte Effect that may or may not be used for your purposes. The Feather Edge effect is not as effective as in Photoshop Elements Feather.
Where you might gain some advantage is the instance where you can create a freeze frame in the video and then apply selectively effects in Photoshop Elements before transferring the freeze frame back as an insert into Premiere Element Timeline content.
Basically you need to distinguish between those effects that can be varied across the duration of the video versus those effects which can be applied to a specific area of the frame. I believe the latter is what you seek.
Please review and consider.
Thank you.
ATR