Can a soldier that hasn't crossed the river yet serve as a cannon's screen just as well as one that has?
These answers restate the rules implemented by our open-source engine, which follows the WXF laws and is regression-tested against the official 110-diagram repetition casebook. Where an answer refers to a specific position, that position lives on the linked reference page, where the build verifies it against the engine before publishing.
Direct answer
Yes, completely equally. The rule for a screen only asks whether exactly one piece stands between the cannon and its target; it does not care what kind of piece it is, who owns it, or, for a soldier specifically, whether it has crossed the river yet. An uncrossed soldier sitting on the right point works precisely as well as a crossed one, an advisor, or an enemy chariot.
Why this is worth stating plainly
Soldiers are the one piece type whose rules change partway through the game, gaining sideways movement after crossing the river, so it is a reasonable guess that other rules involving them might change too. The screen rule is a clean exception: it treats every piece identically regardless of any status of its own.
A common setup that relies on this
A cannon sitting behind its own uncrossed soldiers on the third rank, a standard part of many opening setups, is already using those soldiers as ready screens well before any of them ever reach the river. There is nothing provisional about that arrangement; it works exactly the same as it would with a crossed soldier in the same spot.
Related questions
Does a soldier's screening ability change after it crosses the river?
No, it screens identically before and after crossing; only its own movement options change.
Is there any piece whose screening ability does depend on its own status?
No. Every piece type screens on exactly the same terms, purely by occupying the point.
.