For those who still map for HLDM, I found the following by messing around in the SP maps while on a LAN server.
1. Barnacles, (the apache, can't remember plural for it), turrets,sitting scientists, and dead monsters can be placed in a HLDM map.
2. Scripted sequences do not fire all the time. (I ran across the bridge that gets destroyed in surface tension :D)
3. It's fun to just run around the SP maps when you're insanely bored.
For those HLDM mappers who did'nt know about the monsters, I hope I gave you inspiration.
mp_allowmonsters 1
For example:
give monster_headcrab spawns a headcrab below you and you'll get stuck in it. If want to have serious fun with monsters in HLDM, install Entmod. Entmod has made it a whole lot easier to spawn stuff:
entmod_create monster_alienslave X Y Z 1
EDIT: DO NOT USE VALVE MAPS IN MP!!! I've played them in Frontline Force, The specialists, and with mp_allowmonsters 1. It usually crashes for me. very badly.