Hi billybedeveril
I rarely seen chemical H2S scavenger for gas phase. But i have several experience with scavenger for sour crude. It works fine, and you are right, selection between processing and chemical injection is a trade off between OPEX and CAPEX and it is all case to case basis. Normally what i have seen, Chemical injection is for polishing, just to reduce few ppm of H2S in case of process upset or only if the H2S ppm is quite low.
For gas, the most widely used is chemical absorption (DEA, MDEA, activated MDEA). Some physical-absorption (Selexol) and Phisio-chemical absorption (sulfinol) also available from technology provider.
If you have very high ppm and have enough pressure you may opt for gas membrane for bulk removal and select other process for polishing.I have experience where we H2S by membrane, which than polished further by a reactive adsorption of Solid Bed (granular iron oxide–based bed). In this case in the contract we specify for the manufacturer to take away the spent catalyst upon replacement.