I would like to build an upstream bog. Problem is that I can't find any instructions or guidance on doing this as I guess that it just isn't very common. Or maybe it's a big mistake!

I would like to biuld it about 8' lg x 3' wide and 12"-16" deep. It will be built on a 45 mil liner and be filled with a sand mixture. Plants will be planted directly into the sand medium. It will be sealed without the usual drainage holes at the bottom. It will be fed by a spitter and will create a flow above the soil level which will end up in my 700 gal pond.

Am I asking for problems with this plan?

I seen no real problem with that plan, upstream bogs that use all peat can change the ph level of the pond but sand shouldn't cause any trouble.

I suggest small pea gravel instead of sand. Sand will develop anaerobic pockets where there's no oxygen. In the summer, they may "burp up" pockets of methane and hydrogen sulfide gas. They stick and also can kill aquatic animals. Pea gravel is less anaerobic. Also, you could make an upflow (reverse undergravel filter) filter through the gravel with water pushing up through the gravel for an even better system (but more work). I've never built one of these so this is what I've read.