It would be illegal on a ruckman but it's not illegal to put body on another player around a stoppage.
And if you don't nominate then how do you tell?
The only way it could possibly work would be to change the rules so that no player was allowed to make contact with anyone around a stoppage, and that would be a very strange rule.
Hmm, ok, let's see what the actual rules say.
A shepherd is defined as follows:
Shepherd: the act of a Player using the body to push, bump or block an opposition Player who does not have possession of the football and who is no further than five metres away from the football.
So, we have a 5 metre radius for a start. Plus this rule still does not allow for pushing in the back which is illegal under a different section of the rules.
I seem to recall years ago you could not shepherd the ball, in other words you could only shepherd when a team mate had possession of the ball. I would also say that the above is a quite strange definition of a shepherd, shepherding is blocking, a push or a bump is a push or a bump. I suppose it does mean that players are limited from pushing and bumping when the ball is more than 5 metres away, but they don't actually enforce that and I certainly don't recall it being illegal to bump a player when the ball is more than 5 metres away years ago.
Holding is out since the definition of prohibited contact includes the following:
holds an opposition Player who is not in possession of the football;
The enforcement of that one is laughable.
While I am sympathetic to the reason behind having to nominate a ruck, it is a recipe for congestion. To some extent the amount of time it takes to call a ball up is the problem here, because they let the pack form and muck around for a while, in contrast to what used to be the case when a ball up was called when a pack was forming. They have created the problem and the fix creates yet more problems.
I would be interested to see how it would work if they called a ball up much quicker and threw the ball in the air also much quicker. That may not solve the problem, but we have to find a better solution than nominating a ruck.
Ever since they have implemented adjudication which is designed to "let the game flow" it has flowed worse than ever. The solutions used (don't pay all free kicks, allow packs too much time for a disposal, allow players to hold a player in pursuit of (but not possession of) the ball, etc) have contributed to the congestion problems. Then they try and jerry rig solutions to the subsequent problems, such as the requirement to nominate a ruck. It is a mess. They are making it hard for the umpires and the umpires are not adjudicating the rules very well either.
DS