DD: It's not a silly question and I'm not certain of the answer for a RNAi drug. But with most drugs you need to give enough to reach a therapeutic concentration within the cells of interest. It would matter much less how many virus were in the cell. So I think the answer is that you couldn't use lower doses.
The other problem is that it's an iv drug so I don't think it gets used prophylactically. Brincidofovir is a twice per week pill, so if it shows activity, I'd expect it to be the drug of choice for exposure without symptoms.
To your other post, I think it's a huge deal that TKMR can adapt its drug to the virus' genome. No other drug can do that. ZMapp could be adapted but it would take a long time (months). The nucleoside analogs are essentially not adaptable; it would require a whole new discovery effort to find a replacement.