BTW, my current #5 is an el-cheapo Buck Brothers Jack Plane.
I have one of those ... and have been planning to shoot it as well.
With buckshot.
It's not heavy enough to use as a boat anchor, and it's not flat enough to use as a door stop. Wish I could get more than .05 on 100#s for the steel.
But on to your question:
I think a handle would indeed help with shooting. I tend to get a bit of chaffing between my thumb and index finger (on the webbing) when shooting over a period of time.
However, I would recommend against drilling out your new WR foreplane for this purpose ... use an old beater to test out the principle.
Also the placement of the handle would be critical to its use (and comfort).
I've used a 5, 6, and 7 for shooting (even a 4 on occasion).
Personally I've settled on the 5 due to the right combination of weight and length. (Might also be because I have the most of those and therefore always have a sharp iron available
). Most of the stock I shoot isn't wider than 3" anyway.
I tire after used a 7 for longer periods ... though the added weight is nice. And for some things the length is necessary.
I tend to cramp up and get a lot of chaffing with a 4. Too small to hold properly on its side.
My #6 isn't quite tuned so I'm still on the fence ... as I like it so much for face jointing I wouldn't like to dull the blade by shooting. But to be fair I like the size and weight for the job.
Wait till you get the BU jointer and see how that works as a shooter.
Then decide. You might find that is the answer.