This approach is a great idea. We don't currently use this approach, so I don't have anything to say regarding that experience. But I did have something to add to the conversation.
UD3 DIMENSION
TotalForecast (member)
Base Forecast (member)
FcstAdj1 (member)
FcstAdj2 (member)
FcstAdj3 (member)
You could use one of the UD3 text fields (say txt1) to toggle off an adjustment. In this way, you could remove it from the ending forecast (but have it still available to add back in) with a simple update to the txt1 field and then reconsolidate.
Also, you would have to update the consolidation formula to be something like:
TotalForecast = Base Forecast
If FcstAdj1 Txt1 <> "'exclude from forecast" Then TotalForecast = TotalForecast+FcstAdj1
If FcstAdj2 Txt1 <> "'exclude from forecast" Then TotalForecast = TotalForecast+FcstAdj2
If FcstAdj3 Txt1 <> "'exclude from forecast" Then TotalForecast = TotalForecast+FcstAdj3
This could also be integrated with cube view reporting as well, using showing accounts in the rows and UD3 values in the columns along with the ud3 txt1 value in the first row of data. You could also hide columns that have txt1 values of "exclude from forecast", in the cube view if you prefer.