Mick........I am sure you know the K&S range of brass strip/round/square etc sections from most model shops...& all in 300 mm lengths......you certaily could soft solder these lengths to form a 1200 mm long section.....alternatively you could also silver solder the joints for additional strength.....
Sweating sections with soft solder is OK as you can flood the pre-soldered sections with liquid flux..........joining pre - silver soldered sections is far more difficiult & requires greater heat........low temperature silver solder with a melting point of 450 degrees C is OK....but when this is soldered to a section.........you must provide sufficient heat input to re-fluidise the silver solder .... + add
new or additional borax type flux but at the same time the heat is being soaked into the metal mass.......
...........& we haven't considered distortion yet
Remember when we silver solder we need to remove the flux residue which has been converted to a hard crystaline type deposit
I would stick clear of light sections of brass & any concept of silver solder sweating.........Derek