By request, here's all the dialogue I could find in the subtitles for Republic Commando relating to Darman (RC-1136) and Theta Squad. Within the actual game, there's one mention of Darman's squad (all of whom were killed on Geonosis except Darman) as follows:

Clone Advisor 1: As you already know 36's squad has been incapacitated. Their mission was to disable a droid factory in this sector. Your squad has been retasked for this purpose.

(Clone Advisor 1 is Delta Squad's advisor, the guy who talks to you all throughout the game. (And who has a cool accent! My brother and I giggle about the way he says "Delta"... like "Dee-elta", hehehe. Accents are cool.))

But there's more. It's hard to work out exactly what would have happened had the rest made it into the game, as the dialogue isn't in order and probably covers several contingencies. But here's what I could find. I tried to order them in a logical sequence and postulate how it might have worked in the game. My commentary follows in italics.

Clone Advisor 1: We are planning a two-pronged attack.
Clone Advisor 1: Theta Squad will disable the droid factory. Your team will target the Geonosian leader Sun Fac.
Clone Advisor 1: Theta Squad is well into enemy territory. They're working on blowing a door in between you and your objective.

This probably would have been near the start. In the game, the only briefing you get about your mission is "The squad objective remains: Find Sun Fac and eliminate him." and it seems like more explanation could have been given. (I thought that the first time I played the game too... without reading the text on the loading screen, I'd have no clue who Sun Fac was or why we're trying to kill him.) So it seems there was going to be more explanation here. The "blowing a door" part goes along nicely with the prologue of Hard Contact!

Delta 36: Delta Squad, this is 01/136. I've lost 2 men and my pilot in the crash.
Clone Advisor 1: Roger Three-Six, 38's team is on the way.

Clone Advisor 1: 1136, what's your status?
Clone Advisor 1: Hold tight 36 help is on the way.

Funny that it's Delta 36 here and not Theta 36. But I think they're supposed to be one and the same. It looks like Darman's ship crashed and Delta Squad must go and rendezvous with him. But from another deleted line about how the commandos ride individually on gunships to avoid losing a whole squad (this is implied in the game), it was probably just Darman on the gunship and the rest of his squad is ok. For now.

Clone Advisor 1: 38 proceed to Three-Six's gunship.
Clone Advisor 1: Debris is preventing us from reaching 36's position.
Clone Advisor 1: Proceed through the new opening to 36's position.

Delta 36: Stand clear of the fallen pillar, Three-Eight. I'm going to blast an opening.
Delta 36: Welcome to Geonosis. Three-Eight.
Delta 36: Battle droids are holding a position beyond that fallen pillar.

Delta 36: Take cover!!
Delta 36: Spider Droid approaching our location.

Once Delta gets to Darman's location and clears a path, 36 stays with the squad for a bit. There is a "secure the area in front of the crashed gunship" part on Geonosis in the game... perhaps that's where Dar was going to show up?

Clone Advisor 1: Delta Squad, your brothers in Theta Squad failed their objective.
Delta 62: What happened? Couldn't hack it?
Clone Advisor 1: Heavy casualties sustained watching YOUR back, Delta. So, I'm giving you their objective: Sabotage the droid factory.

This would probably be an alternate (or an additional) line for the one in the game, were the rest of the Theta plot implemented.

Ok, there's my summary of Dar's would-be role in the game. There are some other parts (and funny lines!) that didn't make it into the final version of the game and I'll try to summarise them later.

Oh, and I'm not sure how much of this dialogue was actually recorded, as I haven't figured out the sound format yet. But I'll keep trying to do that as well.