That would be really useful for people who use GWT UiBinder and/or programmatic layout of widgets. This is definitely something we support, but we're angling toward ErraiUI for our demos and stuff. I think with ErraiUI, special row and column widgets for Bootstrap wouldn't be super-useful.
One thing I definitely did run into with Bootstrap was that it's super-awkward to use their jQuery addons in an ErraiUI project. All of the CSS is reusable, but the logic in the plugin is difficult to include and invoke in a natural way. I ended up recreating the popover plugin code in the Grocery List demo: