I've just uploaded a new version (v0.6) that fixes the limitation with extending the selection over open space.
This version also enables complete unconnected open space construction designation as discussed above (so you don't have to be connected to any existing floor tile - you can place it against floor tiles that will exist in the future). I've also enabled this functionality for stairs and ramps, which means you should be able to pre-designate a multi-level structure that the dwarves will be able to traverse as they build the necessary routes.
Like with rectangular select, this feature is not enabled by default; there is a new option, toggled with "o" (Enable Open Placement), that will appear on the construction menu to allow this functionality. This screenshot shows some walls and floors designated with this option turned on, that would not be possible to designate in one go in vanilla (the black tiles are pending floor designations):
While the plugin can manually place construction designations anywhere (which is how I can allow open space designation), it's only possible to invoke the material selection menu by letting the base game do it, which requires a valid tile a construction can be placed on. If your designation includes at least one valid tile, the plugin will use this as an anchor point to invoke the materials menu from. However, when the designation is completely in open space and there are no connections to an already built construction, this is not possible. What the plugin will do is perform a search, starting from the first corner of your selection and spiralling out inside a 11x11 square, going up to 4 z-levels down, to find a valid tile it can use for this purpose. No construction will be placed on this anchor point; it' just being used to invoke the menu.
For this reason, while placing unconnected designations, you may notice when you get to the materials selection screen the cursor has moved to a location that is not in your selection, possibly even on a different z-level if you're designating a multi-storey structure. Don't worry about that, there won't be anything built there. Once you have selected the materials the interface will refresh back at your original cursor location.