I dug around the internet a little more.. Seems the 'official' data on the arbor pin talks about a .222" diameter, so 5.64mm then. A quarter inch would be 6.35mm. I've looked at a few more pictures, and I think the ground flat arbor pin isn't going to be of much use. A hollow replacement arbor pin should be easy enough to fabricate though.
The more I think about it, the more I'm sure this is actually a really easy modification. And that's capital E easy; machining the parts needed shouldn't take more than a few minutes, really. Fine tuning the mechanism could take a little longer, but it's still pretty much a lazy Sunday afternoon project..
Here's a quick schematic view of the parts of the modified arbor pin, and how they go together, both in side view as well as top down view. It's pretty much self-explanatory, I suppose. I left out the spring in my drawing, but you know where it goes..
The dimensions are bound to be incorrect - I worked off the assumption a pin diameter of 6mm would be ok, gave it a 1mm wall thickness. The length of the hollow pin is 100mm, 110 if you include the head of the pin..