I wouldn't know how to make the dialog but the scripting side could go something like this.
1) Place 3 game logics named westrespawn1, westrespawn2 and westrespawn3 in the respawn positions you want.
2) Put respawn_west at the default location you want units to spawn at one of the game logics.
3) In a init.sqs file type the following:
respawnpoint = west1
[] exec "respawn.sqs"
exit
(change west1 to the default one you want to spawn at)
4) A dialog would need to define the variable for each button. respawnpoint = west1, respawnpoint = west2, respawnpoint = west3
5) A script named respawn.sqs should be placed in the mission folder. This is what it should contain:
#start
?!(alive player) : goto "spawn"
~1
goto "start
exit
#spawn
?(respawnpoint == west1) : "Respawn_west" setMarkerPos getPos westrespawn1
?(respawnpoint == west2) : "Respawn_west" setMarkerPos getPos westrespawn2
?(respawnpoint == west3) : "Respawn_west" setMarkerPos getPos westrespawn3
#alive
?(alive player) : goto "start"
~1
goto "alive"
exit
I havn't checked it, but if theres any problems you can't fix yourself then post them and myself or others could try and help you.
If someone else can suggest a better way then please do.