Hi Miltades,
sorry, but I do not (and others, I think too) what You mean.
First, Blender Weight-painting is able to "adjust" the finest possible point on Ur screen, which is the size of one single pixel! Your mentions of the capacities of Blender on this behalf, hypothiseses that You are not really familar with some functions in Blender - or! (that's what I think) You did not tell us the real prob You have.
But if it's something on behalf of "knowledge" I'd suggest to read first something like this:
http://wiki.blender.org/index.php/Doc:2 ... ight_PaintSeccond, all meshes, imported from MH (if they are rigged) are weight-painted, that's right, BUT!! You see always (or better to say: You should see/work on) only on one vertex-group. MH-meshes are exported with a set of predefined vertex-groups for the special "deform-weights" according to the scelleton-structure. So if You weight-paint, You never paint the whole mesh.
Hope You know, how to select the single v-groups in weight-paint of Blender. (if not then use SHIFT plus LM-Button, while hoovering over the region of interest and You'll get the list of nearby v-groups in an pop-up)
The above link leads to the 2.6 Blender docu, but is still up to date in this part also in the newest V 2.71.
Last but not least: I never saw (up to now) those strange combinations, You mention above, so it would be fine to see a screen-shot (at least). (During an unstable nightly-build I saw once someting like You mentioned, but not in a stable and also not in an official release.)
Feel free to ask those questions also in one of the blender forums, cause there are many more people, who are more closer to Blender and Blender-Issues.