virtual fonts in luatex?

["Thomas A. Schmitz" <thomas.schmitz@uni-bonn.de>]

Hi all,

I'm running into a problem, but maybe not all hope is lost. For my  
Greek stuff, I will sometimes need a "dotbelowcomb" accent. Only very  
few fonts have that, but no problem, in mkiv, I can simply take that  
from a follback font:

\definefontfallback [GreekFallback] [name:texgyretermesregular]  
[0x0323] [force=no,check=no]

Now I would like to remap the exclam character to this dotbelowcom.  
Again, no problem: with a fea file, I can write a sub rule

sub exclam by dotbelowcomb

but I can't combine both approaches: if the font doesn't have the  
character, I get an error about "invalid glyph index." In good old  
pdftex, it was possible to define a virtual font for such occasions,  
and I seem to remember that there's something similar in mkiv, but I'm  
unsure how it can be done. Any pointers?

Thanks, and all best

Thomas

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