Hi Rare,
Rectal absorption of drugs is lower than oral absorption, so 100mg in suppository may be roughly equal to 50 to 75 mg in oral capsules, which is a medium dosage.
Keep in mind that 100mg once in a week is not a concern, even orally. This dose may become a concern if you take 100mg daily, and still worst if you take 100mg two times a days for many days. My goal was to put your dose in proportion, to compare with indomethacine doses like 25mg, or 50mg, in order to help compare with what would be needed with other NSAIDS. For example, 100mg of indomethacine would be more equal, in naproxen, to 500mg, rather than to 250mg. ( my goal was not to make you worry about the dosage you take, which is correct ).
The rectal administration may have some advantage in POIS, if we consider that the initial triggering of the immune reaction is in the prostate/genital region. The absorbed anti-inflammatory drug will have higher concentration in this region when given in suppository. To verify if this is the case, we would need to compare the same NSAID for oral vs rectal administration, in order to see if one route of administration is more effective than the other.