Good thread! Welcome, Mr Sheen - a cracker to kick off with.
So in summary, we see that although players can be brought to ground in-goal, the act of doing so does not constitute a Tackle, and so Law 15 is not brought into play. As a result, neither player is required to release the other or the ball, nor are they required to roll away. Neither rucks nor mauls can exist in-goal either, so effectively we remain in open play. The referee is looking for a VERY quick decision on: ball grounded by either team? OR Ball held up? Either way, expect a quick whistle and either a score or a 5m scrum.
No-one has yet given full chapter and verse from the Good Book, so here it is:
15.1 WHERE CAN A TACKLE TAKE PLACE
A tackle can only take place in the field of play.
Law 1: DEFINITIONS
The Ground is the total area shown on the plan. The Ground includes:
The Field of play is the area (as shown on the plan) between the goal lines and the touchlines. These lines are not part of the field of play.
(a) Where can a ruck take place. A ruck can take place only in the field of play.