You could do this with USQL but that would only include completed sessions. You could also filter down to a specific id on the sessions page but could not put this on a dashboard/custom chart.
Right now I don't see a way to do that. You'd need a way to define a new metric similar to calculated service metrics.
You cannot do it right now, as live user sessions are not stored yet in the storage database but kept in memory. It's a subject to change in the near future.
Yeah this can be done via the USQl in the user sessions blade, but its only going to show completed transactions. You could see live ones if you filter based off of the the user name starting with an S but you wont be able to make a dashboard tile or chart.