You could use a tiny Lua script that checks when the map has loaded if it is richland, and sets the cvar to 0 if so.
Something like:
local function richlandtime()
if game.getMap() == "dm_richland" then
RunConsoleCommand("ttt_spec_prop_control", "0")
end
end
hook.Add("Initialize", "richlandtime", richlandtime)
Put in a Lua file (eg. "richlandtime.lua") on the server in the /garrysmod/lua/autorun/server/ directory.
Doesen't work..
ERROR: Hook 'richlandtime' Failed: [lua/autorun/server/richlandtime.lua:2] attempt to call field 'getMap' (a nil value)
Removing Hook 'richlandtime'