What signal loss can I expect by routing a 20 dBm 900MHz signal in a 90° curve in a PCB?
This is probably not too much of a problem at 900MHz - at 10GHz it would certainly need consideration.  The fields on a transmission line like to remain uniform - any discontinuity in the line causes a disturbance of the field pattern which leads to a reactive effect - generally for bends there is a degree of associated 'fringing capacitance'.  A rule of thumb is that the minimum bend radius should be not less than 3*line width.  A corner can also be compensated by use of a chamfer.

Transmission lines are passive and linear, so the loss would not change with power.

I would refer you to Waddell https://books.google.co.uk/books/about/Transmission_Line_Design_Handbook.html?id=MyxTAAAAMAAJ which contains models for microstrip bends

Alternatively Gupta,Garg and Bahl might help - https://books.google.co.uk/books/about/Microstrip_Lines_and_Slotlines_Third_Edi.html?id=1PQfAgAAQBAJ&amp;redir_esc=y - see excerpt below.  However, note the frequencies here and how the match improves dramatically at lower frequencies.

The 1.8W compensated corner is the best, but I have tended to use curves as they are more easily available in PCB tools.  Note that the curve given below is a not the 3*W radius I suggested earlier, but the tight curve shown in (a) - a gentle 3*W bend will perform much better.