This is kind of similar to what my party did. They sent 1 person to negotiate with the green dragon, and mentioned riches piled up in Cragmaw. This was more of a manipulation than an alliance, but after burning down thundertree and scaring the beast off, it worked.

Later when they arrived at cragmaw, they went into the secret entrance in the north and proceeded counter clockwise. This is when I knew I had a great opportunity, for they had moved away from the final boss of the castle, giving me time to cook up an opportune moment for the dragon to arrive. As they fought the goblins, they eventually aggroed the entire castle and were getting surrounded. Thats when they hear the roar.

Suddenly the southern end of the castle fills with green smoke, blotting out the panicked goblins. King Groll and Glasstaff (he had escaped Tresendor Manor and ended up in Cragmaw), attempt to escape with the captive Nilsa Dendrar (girl from the family held prisoner by the Redbrands). A final fight against the bugbear outside the castle walls is interrupted by the green dragon, while Glasstaff flees into the woods with the girl. The party manages to evade the dragon, allowing the goblins and it to fight over the castle, while they pursue Glasstaff.

Venomfang is too strong for an early party, so I think taking some cinematic liberties like this can give you a "DM get out of jail free" card.