Nope, I tried playing alone on LAN and it still wouldn't let me save.
Would it be impossible to just cram all of the maps into one giant map, or is there not enough space for this? It would make everything so much simpler, even if some zones might look a little different due to tileset limitations, this can be worked around with imported custom models of cliffs and custom terrain and whatnot.
EDIT: According to
this calculator, all the maps from the first chapter (including their boundary zones) can fit inside a 288x288 map. This is far from the maximum map size of 480x480, so I think the other chapters can fit in their own giant maps too. What are their individual map dimensions?
The trouble really is the tileset limitations, because only for the first chapter you have Barrens, Ashenvale, Sunken Ruins and Underground, and all make heavy use of their own cliff textures. It's gonna be tough to recreate them using one single tileset. It's a shame the tileset restrictions are still in place in reforged, texture memory is not a limiting factor anymore, Blizzard could have easily allowed us to use ALL of the tilesets in one map.