No, this would not work.
Slow start doesn't count as a stat lowering. (as in it doesn't count as one of the stat changes caused by moves that lower or raise a stat by 6 stages) It doesn't show the stat lowering animation when Regigigas comes out. It just halves the attack and speed stats, much like Huge Power, which boosts the attack, but it doesn't count as one of the 6 stat change stages.
Source: Using Regigigas in game
I hope I helped!
EDIT: the 6 stat change stages are:
For raising a stat, +1 (the first stage) is the stat multiplied by 1.5, +2 is multiplied by 2, +3 is by 2.5, +4 is by 3, +5 is by 3.5 and +6 is by 4.
For lowering a stat, exactly the same as above, except the stages have - before them, and they are divided by 0.75, 0.5, 0.25 etc.