On GPON or XGSPON, sending 1490nm on the upstream will have no effect unless you get to incredibly high power levels where they cause damage to the transmit laser on the OLT's optics for GPON (many dB higher for XGSPON). Fibre optic cables and PLC splitters don't reflect signals at significant levels (directivity is typically >= 55dB). I have unintentionally tested this by accidentally connecting 2 different GPON segments together in a splitter cabinet; nothing broke.

On GPON, 1310nm light launched at 2-8dB and 1270nm light at similar power levels on XGSPON is enough to blind the OLT from being able to receive signals from other ONUs. Risk-wise, it's the same as cable modems that can spray noise over the upstream channel. But most people aren't interested in launching DoS attacks that take themselves offline.