I'm pretty sure that to get 1 Watt you have to use direct sequence or frequency-hopping spread spectrum under FCC part 15.247... Just sitting on a frequency as a 15.249 device or general ISM device you aren't allowed to use much power at all.
From my reading of the rules, you are limited to broadcasting +36 dBm EIRP (30 dBm [1W] into a 6 dBi antenna -> 4W EIRP). This can be a single frequency. In the 900 MHz band, your bandwidth is limited to 902-928 MHz (US limitations, different in other global regions).
You can happily beam out a 915 MHz single-tone CW carrier at +36dBm EIRP all day and that is fine.
The reason your gen2 reader is frequency hopping is not FCC mandated but protocol mandated. The UHF RFID gen2 tags have a high manufacturing variation and the RF frequency response varies greatly between tags. Some tags may be resonant closer to 902 MHz and some tags may be resonant closer to 928 MHz. Hopping over various frequencies allows the reader to address all tags in its view.
The only reason the readers are "required" to do hopping is to conform to the gen 2 protocol. In fact, from what I remember you can use the LLRP (low-level reader protocol) to stuff the frequency hopping table with a single frequency so that it will stop frequency hopping.
(My PhD was on high-data rate (up to 100 Mbps) rfid/backscatter communication.)
This is what I initially thought as well, having built an ISM band transceiver around the 350/400 MHz bands a few years ago. I recalled much lower power limits.
(I had posted something along the lines of "there's no way in hell the FCC would let you transmit at half a watt" but have since edited).
Apparently the 900 MHz bands are different, and the FCC has rules governing output power going into the antenna (1 watt), as well as an effective radiated power (which essentially places limits on your antenna gain) (4 watts).
Yup, but we can 868MHz in Europe, and with all the support from international backers I think we may end up doing that as a stretch goal! Just have to talk it our with our RF expert, which we're doing!
Hadn't seen the rules for 900MHz though.