This will eliminate many inclusion impurities (butane and target chemicals [cannabinoids] existing in a lattice structure together--as in solution)--however it will greatly increase the amount of occlusion impurities (trapped pockets of butane within the [solid when hard] cannabinoids).
Occlusion impurities tend to be more significant than inclusion impurities, in terms of molar quantities (but this will depend on the particular solvent/solute). This is especially true when comparing occlusion impurities vs. inclusion impurities when obtaining an oil via a thin-film evaporation (a special kind of "purge").
If the extract were pure THC or CBD, the occlusion impurities would be more predictable--because there would be an ordered, predictable, lattice structure for a butane and THC binary system. However, there are many compounds present, and there are many possible interactions and molecular states/configurations to consider. Due to the water solubility of butane--there are likely to be clathrate structures present as well--which can greatly increase expectation for occlusion impurity.
Alright so to answer a few questions this might bring up more succinctly:
1. Will this work?
Yes, it will--but there are some caveats. When you pour butane onto dry ice, it will bubble furiously , this will be a huge pain in the ass in terms of collecting your material. You will have to work with extremely small amounts of extract liquid and extremely large amounts of dry ice to have any hope of evaporating the butane before the CO2 fully sublimates. The sublimation point of CO2 is -78deg C (~ -110F). The boiling point of butane is -1C (~ 30F).
2. Will it be better quality in terms of butane content?
Probably not, but there is no definitive answer on this.
Instead of pouring the extract liquid directly onto dry ice, you can create a dry ice bath for a glass vessel (something which will withstand very cold temperatures) by mixing dry ice with any number of solvents with low freezing points (ethanol is common). This will achieve the same effect you are looking for but without direct contact between the CO2 and extract liquid itself.
When creating the bath, you will see why pouring nonpolar liquids containing compounds you wish to retain directly onto dry ice is a terrible idea.
It is important to note that condensation may become a significant issue when using this process--and also important to remember that butane does not have zero water solubility.
