OK, this is what I found, C42 is a 1000µf @ 10V, its + is connected a bottom leg of D6. D6 is connected to the top of D5, and the bottom of D5 goes to UJ18. I think its part of the start up circuitry, but I do not have a schematic to verify it. C42 looks like it is connected to the top ground plane on the - side but it is not. Be careful not to short it out here when you replace it.

My little girl recapped my IIfx while I was away, and she did a great job. It's C29 and C4 (I think on both) that are the leaky caps that plague Macs all over - the 47µf @ 16V. Trag go me a couple of Black Tantalum Caps to recap the board and and they look great! Since the rest of the IIfx use black caps, these two look like they are right at home!