How to validate the use of an anti-virus monoclonal antibody in IHC by spiking a fresh organ with infected cultured cells?
Injecting into a fresh organ seems like a needless complication if you're just validating your antibody and not interested in gathering data about the interaction between the cells and the organ. (In fact, if you're using anything more complex than a flatworm I'd hope your IRB would have problems with needlessly sacrificing animals to get those organs.)

It seems like your purpose would be better fulfilled by making an FFPE cell plug and doing your IHC on that. Here's a protocol for making cell plugs:
Johns Hopkins Tissue Microarray Lab Cell Plug Protocol