A coating rod or brush works pretty good for me (depending on the paper). Most of the sensitizer is not used for the image but will be washed away. An inkjet printer could deposit sensitizer like it normally does it with ink or pigment to print an image. That dried sensitizer image will be exposed uniformly to UV and developed. One could have different "inks" which is sensitizer in different concentrations like normal printers use different black/grays but also different other metals (e.g. gold) to modify color. Only as much Pd/Pt would be used as the image requires. This would be not contact printing but digital printing.
I would not see a principal problem spraying the sensitizer except it will dry and crystalize in the print head. Flushing and priming could waste a lot of sensitizer.
Also it would require a lot of printing software development (so not for me). These prints will be as stable as any Pd/Pt print but would it generate similar tonal range that traditional pd/pt prints have?