Real Time SQL Server Analysis Services Cube Processing Report
I guess you are worried about the downtime due to cube processing. 
As detailed in the post below, the downtime, due to processing, is very minimal . 

http://www.jamesserra.com/archive/2011/06/can-you-query-a-ssas-cube-while-it-is-processing/

In the comments section, there is an approach mentioned by which you can reduce this downtime (with few caveats) by using one server to do the processing and then sync the DBs. 

If you still want to do the report, you can follow the approach suggested here:

https://stackoverflow.com/questions/11899250/how-to-determine-which-ssas-cube-is-processing-now