Do your graphics on a low resolution 3rd screen, then scale it to fullscreen.
Edit:
To be preciser:
- use CREATESCREEN screenID, spriteID, 320, 240 to create a virtual screen with the resolution of 320x240
- call SCREENSIZE screenX, screenY to get the current screen width and height
- before drawing your game stuff call USESCREEN screenID
- after drawing your game stuff call USESCREEN -1
- then call STRETCHSPRITE spriteID, 0, 0, screenX, screenY
Be happy with blocky retro graphics at FullHD
