type of mod? chip? which kind?
why would you always want to boot to the ms dash? if your trying to keep your xbox from connecting to xbox live when others use your box you could just put passwords to stop others from using it. please expand on "why" you always want to boot to only the ms dash.
but to answer your question, if you have a chip,(don't know about soft mods but i would assume yes as well) yes you can always boot to ms dash if you push either button.
all you would do is alter your bios for the eject button and the power button(eject button with a chip always boots to ms dash) to always boot ms dash bios.
if you wanted to use the "A" button to boot to a dash(evox x) you would edit those bios to look for that dash.
look for bios editors here
** a word of warning** if you edit your power button to boot to the ms dash technically your chip may still be on(need to know the type of chip you have) the full proof method will be to edit your bios,boot them and "if" your chip has a light that is on when the chip is on and that light is lit when your ms dash bios are loaded(power button).... you will be at the ms dash but your chip will still be enabled and if you aptempt to log into "live" you will be banned so make sure to read up before getting yourself into a expensive situation.
We can help you further if you need help but please add as many details as you can so we can help you the best ways possible feel free to contact me threw msn messenger if you need help.... i just bought a really expensive rice burner and i don't know when ill be active here so...... msn messenger is alway your best bet with me.... hope others can chime in
this might be exactly what your looking for.......read this