First of all, I think you made a mistake in question. The Regis have 7 dots not 6.
Now to the question, no the dots on Regigigas does not signify the other regis (as far as I am aware). There are 6 regis (including regigigas) and there are 7 dots.
It signifies the braille writing system. In gen 3 games you have to decipher braille codes in order to get the Regis. However, the dots or the letter or the word (its unclear) does not appear in the braille language. It maybe a signature decoration for the Regis. Remember the first three Regis first appeared in gen 3 and not all together. So the creators planned the next Gen 8 regis 15 years ago? Possible but a bit to take in.
Maybe in future there will be one more regi to complete the theory. But for now, no.