If they have to escape to Japan, then they're hardly of the status to intermarry with the Imperial family. At most they could give one of their women as concubines (or perhaps have a son marry the daughter of a low-ranking prince) if they managed to gain a reasonable position at court. This is akin to Baekje who could marry among the House of Yamato because they were a valuable ally, but when they collapsed they were no longer able to and their princes and elite joined the ranks of the provincial and court nobles.
And given Baekje's destruction--at the hands of the Tang no less--despite a huge amount of Japanese aid (Battle of Baekgang), I doubt Japan would be very interested in intervening on the continent. Baekje was a long term ally--losing Baekje theoretically would directly threaten Japan. But China is distant and reinstalling Tang doesn't benefit Japan. And by the time Tang needs to be re-installed as a power, there are already pro-Tang warlords in China. Japan also may have trouble finding the manpower to send a large expedition since they'd have to strip thousands of soldiers from places they're needed like keeping the Emishi subdued.
It's an extremely hard political goal for little gain, and one highly unlikely to succeed given none of the actual attempts to restore the Tang Dynasty in the 10th century succeeded.
