Well, the question is highly explanatory, and if my memory serves me right, I remember seeing another old topic with the same doubt, but I ended up not finding it…
There is. As FE6 through FE8 all run on GBA, you can copy the functions involved with each’ opening intro, insert and call them using procs. I’m not quite sure if the proc system between the three games differs much, but you should be able to mimic FE6’ and FE7’s opening in FE8 just fine with its variety of proc codes.
If you haven’t yet, I recommend checking out Teq’s tutorial on assembly and Stan’s doc on 6Cs (procs).
EDIT: Going through Stan’s LUA script, it does seem that there are no big additions/deletions/changes to the proc system between FE6, FE7 and FE8 (I haven’t worked with LUA before though). Stan does mention that instruction 0x15 is used for the name field for procs in FE6 and FE7, whereas FE8 uses 0x1.
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Holly Naga! I thought my question would hardly have a solution; I can only say that I am extremely grateful for the response and help.
PS: I’ve seen your work, and I’m in love with your Custom Chapter Intro and the opening of your “Subtitle” project 
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