Is it possible to use spike-ins with droplet technologies?
I believe it is mostly a cost issue as spike-ins are "expensive" to use given the large number of cells that are/can be sequenced via droplet based methods (as compared to plate-based ones). The 10x link that you provided actually states that this is possible with 10x and Klein et al. used ERCC spike-ins in 953 droplets (Cell. 2015 May 21;161(5):1187-1201. doi: 10.1016/j.cell.2015.04.044.).