Hurrah! I managed to solve an interface issue that likely has all of you staying up at night (okay, probably not): how to easily create seamless tile patterns for all tiling types.
The solution involves allowing the creation of "Guide points" on the boundary guides. Whenever you create a guide point on one of the sides, copies appear on other corresponding sides, so you can just edit your original object for another path to link to the point.
Guide points can be moved around, and their copies get moved along with them.
What's more, if you shift-drag on a guide point, a segment appears on both the original guide points and its copies, so you can use them as angle guides to create smooth transitions.
It gets better: you can break the tiling with a "fuse path" method. When that happens, all the tiles are joined by the nodes located on the guide points! http://postimage.org/image/ae0dae6mh/
You can also create "guide point pairs" that allow fusing objects instead of paths instead, so the fills behave correctly. http://postimage.org/image/kqmnwh05l/
Now even I can easily create seamless P3 and P6 patterns!
Anyway, I now have everything in a dedicated wiki page: http://wiki.inkscape.org/wiki/index.php/Tiling_tool
It's a pity image upload still don't work. I have the toolbar designs available now too: http://postimage.org/image/9w57ujh1l/
The only big thing left for me to do is to figure out the work order for Clones Along Path. I want to integrate Pattern Along Path (Repeated and Repeated stretched) into this mode (Bend and Pattern Along Path: Single modes can be integrated into a sub-mode of Transform or Node instead via the ability to add a control spline to an object).
But the problem is, when a Pattern is applied to a path it's deformed, so editing the original unit becomes a bit problematic. I'll probably figure out an interface for that eventually though.
In the meantime, thanks for everyone's input, I welcome all further comments so that we can work towards a blueprint that we can post out there in hopes that someone on day will eventually get interested. Anyone wants to make Inkscape the most easy program to create seamless tiling for all 17 tiling options? :)