Now I wonder if it possible to create an SQL VIEW in any of the popular RDBMSs that would pack flags in one int8, but would expose them as separate columns in the VIEW in a way that would make not just SELECTs, but also UPDATEs and INSERTs, work.

That’s ubiquitous in slightly lower level programming (pretty much every non-small C program probably does it in some way or other), but the problem with doing that in this database scenario is that you might run into atomicity issues.

To toggle a flag, you have to read the column, modify it, and write it back. If someone else does that to another flag at the same time, there’s a race that one of you might win, at the expense of the other flag’s new value.

Unless you employ a lock that would otherwise not be necessary.