I'm going tobe doing an 80km offroad race next year, on the CX bike and it's a fairly demanding course. I don't want to take a pounding for 4.5+ hours to be honest. I can handle it onthe road but probably not offroad (I don't own an MTB anymore)

Has anyone used these or something similar in the way of suspension on a CX bike for longer riding or just in general? If so, what?

Any responses welcome.

Currently riding an older 2nd hand red 2009 Trek XO2 with carbon forks btw.

Those look like tanks. I will concede that for the comfort a suspension fork would provide, an extra 1000+ grams could be worth it, but have you thought about just lowering your tire pressure? The X02 is an awesome bike, I would be sad to see one of those forks on it. If you were riding a Salsa Fargo or another adventure touring bike on this ride, a suspension fork would be appropriate, but suspension just seems out of place on a cross bike to me.
That is only my opinion though. Could you tell us more about the course? How off-road is off-road here? Are you talking about an 80k gravel ride or 80k of rocks, logs, and single track?

It's 95% single track with about 2km of the 80km being gravel road and 2km being sealed road. About 4+ hours of riding.

I would only be thinking of putting these forks on for this particular race and one other, they'd come off for the rest of the season for actual cross racing. I'm not worried about weight here, I'm worried about my back and the rest of me!

4 hours of this sort of single trzck riding, def. MTB instead of cross

http://contour.com/stories/59626

If you are determined to do a singletrack race like that one on your CX bike instead of a MTB, you will need some sort of suspension fork. This looks way too rough for a CX bike, but that might soften it up a bit.
Out of curiosity, what wheel/tire/tire pressure combo will you be riding on for this race?

Not sure, something low pressure and squishy. Maybe Maxxis Raze as it's pretty smooth surface. My friend did it last year on a Beone full carbon CX with 404s but he was pretty smashed at the end of it!

I would stick with the rigid forks and ride with 38mm or 42mm CX/trekking tyres - cheap forks add weight and the constant 'clunking' and watching them waggle under braking would get to you after a while.

_________________
Just ride