In actual speech this is more likely to be heard as Dych chi'n 'neud brecwast?, I believe.
Could you use this in the future sense? I.e. If you get to a hotel at night, could you ask this to imply 'in the morning?'
Yes, you could: Dych chi'n gwneud brecwast fory / yn y bore? (...tomorrow / in the morning) or just leave the time phrase out altogether.
Thank you for your expertise. So, is it correct to write "beth dych chi'n gweneud ar gyfr brecwast"?