Did Beleg and Mablung participate in the Dagor Bragollach?

[Tyelkormo the Fair]

Did these sindarin captains have any parts in this battle like they did later in Niraneth?

 

[Tar-Elenion]

Forestalling the inevitable?

Unlikely except in such circumstance as the forces of Morgoth were near to Doriath.

"The most part of the Grey-elves fled south and forsook the northern war; many were received into Doriath, and the kingdom and strength of Thingol grew greater in that time, for the power of Melian the queen was woven about his borders and evil could not yet enter that hidden realm."
"War ceased not wholly ever again in Beleriand; but the Battle of Sudden Flame is held to have ended with the coming of spring, when the onslaught of Morgoth grew less."
"Thence they passed over Gelion with fire and terror and came far into East Beleriand"
Silm. Ch. 18

 

[Vorondil]

That's a good question Aredhel, and I have no clue as to the answer.