On my system it appears that the modprobe from the speakup.synth=synth_name is
working except if this is meant to emulate the speakup kernel option (if speakup
were built in the kernel) then synth_name differs from the module name slightly.
I believe the difference is that the module name is the synth_name with speakup_
prepended (eg. for the apollo synthesiser I give speakup.synth=apollo at the
boot prompt but the module name is speakup_apollo, currently it seems like
grml-autoconfig is using the synth_name as the module name, so in this case
loading module apollo).

So either this could be left as it currently is and users need to specify the
full speakup module name (eg. speakup.synth=speakup_apollo) but I would
recommend changing the option name to avoid confusion with the built in speakup
standard (of course document this to make it clear) or add the speakup_ prefix
to the synth_name given. I would prefer the latter as it then fits with the
"usual". If doing the latter is there intention to support the other speakup
kernel options? (eg. speakup.ser, speakup.quiet, etc)