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It seems that continuemove does not terminate program execution, which is sort of what I expected from the description ("returns to the form for making a move"). But any code that is after it will still be executed, if you don't take care to disable it in an else-clause, or in my case by putting a return; behind the continuemove. So it has a delayed effect rather than an instant one. Perhaps this should be made clearer in the manual.

P.S.: It would still be nice if you could put this in the JavaScript for the continuemove page:

function F() {
  if(document.getElementsByName('pastmoves').length == 0) return;
  if(legalMoves) {
    var ori = legalMoves[0][0];
    for(var i=legalMoves.length-1; i>0; i--) if(legalMoves[i][0] != ori) break;
    if(!i) movePiece(ori);
  }
}
setTimeout("F()", 200);

This will automatically highlight the continuation moves if there is only one piece that can make them. But only on a page that contains a 'Past moves' field. I tested this code on a local copy I made of such a page.

[Edit] BTW, how about expanding this code to allow a GAME-code-specified text to appear in the move entry field, in the case it 'auto-selects'? In some variants, entering a locust capture as two normal moves, first to make the capture, and then to move on to the final destination is quite unnatural. E.g. in Odin's Rune Chess the Forest Ox moves like a Knight, but can optionally make one of the enemies adjacent to its destination disappear. The Natural way to enter that is through move + suicide, e.g. N g1-f3; @-e4. The alternative, N g1-e4; N e4-f3 is somewhat counter-intuitive, because of the strange first leap. There doesn't seem a way to enter the suicide notation with the mouse, though.

This problem could be solved by having (after entering N g1-f3, which would be recognized as ambiguous because the optional suicide might follow, and would thus do a continuemove) the form for the follow-up move appear with " @-" in the move-entry field, and all pieces that could be optionally captured highlighted. Clicking on one of the pieces would then provide the destination square (e4, say), and send the move as @-e4. If the user doesn't want to make the capture, he could hit the Pass button.

One way to achieve this would be to allow the GAME code to do setlegal 0 " @-"; to append a fake move to the $legalmoves array, which would be passed to the JavaScript as [0, " @-"]. The JavaScript could recognize that from legalMoves[i][0], and remove it from the array, writing legalMoves[i][1] into the move entry field, before calling movePiece to highlight the destinations given in the remaining legalMoves.

The GAME code could then also use this for facilitating entry of a freedrop, e.g. for a non-standard castling where the Rook does not have to end up next to the King, but could go on any square the King moved through. After entering the King move the continuemove screen could then be made to pop up with " R-" (or " r-") in the move entry, and the possible Rook destinations highlighted, by using setlegal 0 " R-"; before the continuemove. It would also facilitate the entry of moves for the Valkyrie in Odin's Rune Chess: when this 'captures' a friend, the latter can be relocated to any square the Valkerie passed through (or came from). The GAME code knows what the first leg captured, so it could prepare the entry of the freedrop of it.

Some cosmetic issues:

when I use the Play function in the preset's menu, I get to see this:

This looks a bit disproportional. The move entries look excessively large, while the message I am trying to show to the user (arguably by far the most important item in this screen...) is printed in a miniscule font above those, hardly noticeable at all.  Also, there is a strange repetition of the 'past moves' in a much smaller size left of the 'Turn' field, the purpose of which is not clear.

When I use the 'Move' mode of the preset, the layout looks a bit more balanced, but the message to the user is still very inconspicuous:

It seems more logical to me if the specific message to the user would be appended to the header, like "Move as White: enter second leg or pass". Or in the second case "White to play: enter second leg or pass" in a much larger font.