Making a poolish/biga from malted flour?

I've never worked with malted flour, but if it's made of malted grains, there will be a higher sugars content in it, which means the starter/yeast will have much more to feast on. It will start faster, might be more runny than the regular flour one. It should work, but if you use malted flour for the whole bread, it might provide different effects than the regular poolish.

I'd say the only way to find out is to try and let us know what happened. Then adjust the recipe the way you'd prefer it to be and enjoy the bake :)

I think I would be more inclined to make the poolish with bread flour, and make a soaker of the malted flour(s) and seeds, then mix them together in the dough the next morning (or later that day). That way, if you are happy with the way your poolish or biga performs now, you don't have to change anything. But as tptak says, try it and see! Learning about these things is half the fun of bread baking. :)

Malted flour in the UK may mean what we call sprouted flour here in the USA where the grains are only sprouted for less than 24 hours and have not developed the diastatic power of malted grains that are sprouted for 4-5 days and used to make white diastatic malt or red non diatstic malts.

Sprouted grains are only slightly more diastatic so you can treat them like any other flour in any bread way with the note that things will likely go a bit faster than normal and the results will likely taste better with more depth of flavor, brown better, rise better, be a bit more sweet and much better for you nutritionally.

I use them a, the time in preferments and bread flour for these reasons.  Peter Reinhart says sprouted grains are a bread revolution - the name of his latest book so I take that with a grain of salt but my own experiences with sprouted grains are quite revolutionary for me in many ways - the thing is that sprouted grain breads have been around since the ancient Egyptians...... so how revolutionary could they be today:-)  There hasn't much new in the bread world for a very, very long time it is just that some of it is being rediscovered by today's bakers after being long forgotten.

Happy  Baking  with Sprouted Grains