With Gokaiger they got lucky that Teruaki Ogawa did the toku hero twitter account after the earthquake and encouraged former Sentai actors to take the initiative and get back in touch
I don't mean to be mean, what Ogawa did with the Twitter account was nice and everything, but...I'm tired of people twisting what the Gokai producers said and giving him the sole credit for the guest appearances. (Guest appearances were already going to be a part of the show, just in a smaller capacity, so how's that even work?)
It was Red Ranger who brought this news to these forums, and if you read carefully, the producers did mention Ogawa's Twitter, but that wasn't all.
http://forums.henshinjustice.net/showpost.php?p=1577397&postcount=28 "They started receiving lots of calls and messages from actors and even retired guys who found out about the show through the toku twitter/
other means to let them show up in Gokaiger."
Basically, I feel that ANY Sentai actor who had an online presence (by either blog or Twitter) was already aware of Gokaiger, if not on their own then by fans who messaged them about it (asking if they would be on the show). Like, I know Yellow Lion and Black Condor's actors had been aware of Gokaiger right from the first episode -- they ended up being two of the show's best guests and it had nothing to do with Ogawa or his Toku Hero Twitter. (Yellow Lion's actor had attended the Gokaiger premiere at Tokyo Dome with his son, meeting staff backstage and putting in the word he wanted to come back. Condor's actor watched the first episode and posted on his message board his displeasure with Condor seeming alive and well in the Legend War.)
Producer Utsunomiya also once credited director and Sentai fanboy Noboru Takemoto for deciding who to bring back. Takemoto is often a guest at fan events, and pretty much everyone who guest-starred in Gokaiger is a regular of those events (especially all of the people he rounded up for Sentai 199). Now, who's more likely to get people involved -- one of the show's directors or a Twitter account that not everyone who did the show participated in? Takemoto deserves a lot of credit.
I also think that maybe a lot of people started to be more aware of Gokaiger after the 199 movie, seeing the old people in the advertisements, and THAT prompted some to get in touch with Toei. The movie hit around the same time as Ogawa's Twitter was taking off, so it could easily be what got a lot of old actors' attention.