While in a structure you have "borrowed" blocks from, there would be a yellow number at the bottom of the inventory slot indicating how many blocks you have borrowed. The total number of blocks would still be displayed at the top.
While in a structure while having borrowed blocks from another structure, the number at the top of the inventory slot would be yellow (as opposed to the usual white.) The number shown at the top would be subtract the number of blocks that cannot be placed on the current structure. Once out of blocks the number would turn grey, and show how many borrowed blocks there were.
Astronaut mode would be similar, except that it would show only the blocks that have not been borrowed from everywhere in yellow at first.
Also, my "borrowing" idea is an extension of your BV concept. If you place down 1000 shields, then the BV increases by 1000, and you can remove them. The "borrowing" is a buffer, so if I want to take blocks and put them elsewhere I can, without having to go through first and salvage off an area. Basically with this BV wouldn't interfere with upgrading stations. It would require workarounds to deal with planetary construction, but it's more workable and won't deny editing. In addition, it won't add economic leakage.