I remember this being asked before. Back then, I tested a bunch of different possibilities but none of them were successful. As a result, I don't think this is coded in AM like the artwork/image files that allow for a "default" when no match is found based on the Display or Emulator name.
I believe a work around would be coding your layout.nut to contain an "if X then <game>.txt, else show default.txt" type of statement. I haven't tested this but there's no reason it shouldn't work. You can see an example of this in the H.U.D layout here
http://forum.attractmode.org/index.php?topic=2470.msg16848#msg16848.
Here's the section of code. (This isn't the default H.U.D. code but my variation since the original H.U.D. 1.0 used romlists for information instead of the builtin Overview feature. I believe this has changed in the H.U.D. layout since then.)
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
//
// #EXTRA
//
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
function extra( index_offset ) {
local text = fe.game_info(Info.Extra, index_offset);
if (text != "") return text.toupper();
else return "MISSING EXTRA";
}
local extra = fe.add_text ( "[Overview]", flx*0.01, fly*0.26, flw*0.21, flh*0.76 ); // Max 1040 chhudacters
extra.align = Align.TopLeft;
extra.alpha = huda;
extra.word_wrap = true;
extra.margin = 0;
extra.charsize = flh*0.020;
extra.line_spacing = 0.95;
if (bg == 1) colors( extra, "orange", 2 );
fe.add_transition_callback("extra2");
function extra2( ttype, var, ttime ) {
if ( ttype == Transition.ToNewList || ttype == Transition.ToNewSelection || ttype == Transition.FromOldSelection ) {
local text = fe.game_info(Info.Extra,0);
if (Overview= "") {
if ( my_config["font"] == "Barlow") extra.font="BarlowCondensed-Medium";
else if ( my_config["font"] == "Barlow-Bold") extra.font="BarlowCondensed-Bold";
else if ( my_config["font"] == "Hacked") extra.font="HACKED";
else if ( my_config["font"] == "SFPixelate") extra.font="SFPixelate-Bold";
colors( extra, col, 0 );
extra.charsize = flh*0.020;
}
if (text == "") {
extra.font = "BarlowCondensed-Bold";
colors( extra, miss, 0 );
extra.charsize = flh*0.055;
}
}
}
I know it's not what you're hoping for but maybe it can point you in the right direction.