I happened to watch the following video today and was interested to hear how the Democratic Progressive Party was involved in the 1987 movement to allow Mainlanders to return to visit family there. Most accounts that I've read, like that of Murray Rubinstein, depict the opening up of opportunities to visit family simply as one of Chiang Ching-kuo's reforms. Other accounts, like that of Shelley Rigger, emphasize the effects of that policy change.*
This video depicts in more detail the process that veterans went through to gain the right to visit relatives in China. It describes the veterans' agonizing desire to know what happened to their families. As DPP official Yu Shyi-kun, who was a Taiwan provincial assemblyman at the time, says, "In addition to not being able to see family members, they couldn't even write letters to their relatives. So no one knew if they were dead or alive. Can you think of anything crueler than this?" And as veteran Liu Minguo says, "Soldiers have to listen to orders. ... In the military, if you're ordered not to do something, you can't do it. If they say it's white, it's white; if they say it's black, it's black." But according to the video, these soldiers (who became veterans) had to internalize their pain because they knew it was illegal even to express these feelings. (I've written before about how for some soldiers, such pain led to suicide and even murder.)
According to the video, members of the dangwai ("Outside-the-(KMT)-party," which later became the DPP) decided to help these veterans try to contact their family by allowing them to send letters via their magazine, Progress magazine (前進周刊), and through the mailbox of then-dangwai legislator Xu Guotai. The program says that they helped send 300-400 letters.
After the veterans formed a "Association for the Promotion of Mainlanders to Return Home to Visit Relatives" (外省人返鄉探親促進會) and took to the streets to ask the KMT government to let them visit their families in China, the DPP voted to support the Mainlanders' attempts to return home. DPP politicians persistently asked KMT officials to allow the veterans to go home. But the KMT, most importantly President Chiang Ching-kuo, was afraid that such a policy would play into the PRC's plot to reunify under the Communists.
The veterans began to take to the streets, carrying signs, handing out leaflets, and organizing speeches to let Taiwanese know about their pain and to pressure the KMT to change its mind. On June 28, 1986, a meeting of the Association held in Taipei attracted over 20,000 supporters and officials from the KMT's intelligence bureau. The veterans' tearful songs about going home to find their mothers moved the audience to tears. At this point, according to the video, the veterans' tears and song were finally heard by Chiang Ching-kuo. In October of 1987, the Executive Yuan declared that Mainlanders with family in China could return to visit their relatives.
In the video, Yu Shyi-kun speculates that because the DPP was promoting this policy, Chiang Ching-kuo became concerned that if the KMT didn't pay attention to the veterans' request, it would lose the support of some of its most loyal followers. Tamkang University professor Chang Wu-Ueh (張五岳) agrees that the support of the DPP was vital to publicizing this issue and pressuring the KMT to change its policy.
* I did find a footnote in this 1999 article by Yu-Shan Wu that cites a 1998 book by Kuo Cheng-liang (郭正亮), 民進黨轉型之痛 (The DPP's Ordeal of Transformation, or as Wu translates it, The DPP's Agony of Transition). According to Wu, Kuo argues that "there was a period in the late 1980s of far-sighted pragmatism in the DPP's attitude toward the Chinese mainland. At this time, the DPP exposed the rigidities of the KMT's mainland policy, championed open communications with the mainland, and hoped that by unilaterally recognizing the PRC they would encourage Beijing to respect Taiwan's sovereignty" (568 n. 6).