thanks for posting your image, .. this is mindblowing !

probably not much help but...
burning / dodging the individual 35mm frames sounds
like something that might be insane, but a long wire - lollypop and a paper with a hole
might work, do it in stages with rubylith covering the stuff you have worked on
and are going to work on and do the varied exposures one at a time
and THEN layer on the complimentary light that will cover the whole image
(not sure if that makes sense )
.... if the exposure of the whole image is 15 seconds but the denser frames need 1-2 seconds less more, and the
thinner frames need 1-2 seconds less light do all of that 1 at a time .. then do the light they all have in common ( 13-14 seconds )