Inkscape

Tweak tool works, too. I tried it yesterday ;-)

Oh well. Needs some more testing.
Although I have the same problem with the tweak tool -if you lighten a dark fill or vice versa the saturation will be low.

Anyway, is it possible to change one segment in the mesh from curved to linear? Can see the icon but couldn't get it to work.

It worked for me yesterday (with a fresh revision from the 0.92 branch). I think it was necessary to select two adjacent nodes with Shift+Click / box selection, then I clicked on the button with the straight line segment on it. The funny thing is that the button *toggles* between bezier and line, the icon is a bit misleading... Click again to show handles again.

Downloaded the latest(? at least which is on the download link, Inkscape 0.92pre3 15195) and couldn't get the tweak tool to work.

That one is too old :-( The tweak tool thing was introduced in 15197 (http://bazaar.launchpad.net/~inkscape.dev/inkscape/0.92.x/revision/15197, Nov. 17th)
I also couldn't find a current precompiled trunk version for you to try, so this feature is not yet available for Windows users who don't compile.

Will the tweak tool work for nudging mesh nodes as well?

Currently works on the full object, just like for any other object. Don't know about any plans, sorry.
I've made a feature request for node sculpting to work on mesh gradient stops a couple of days ago, that's a bit similar in functionality (but less random). I found it difficult to move nodes around in ways that make sense - if it worked just like the node tool does, but on the mesh nodes, that would make editing a lot easier and more intuitive.

Most of the 'color' extensions seem to work, too :) (but sometimes in unexpected ways, some need to be applied twice to work?...)

And now they stopped working... Guess I might have been the first to try this out.