Use diode to drop down voltage from ~3.7V for nRF24L01
So a Li-ion battery typically charges to 4.2v. You would have to look at the Vf vs I curve for the specific diode to see if it dropes the voltage enough. Based on the current you mentioned, with only 1 diode in series it would be very close to the upper limit, so you would really need more like 2 diodes. That being said, LDO regulators are cheep for only 100mA, it really would be a better solution. Also keep in mind if you are charging the Li-ion battery in the circuit, the charge voltages could be higher than 4.2V depending on the specific charger. 
A similar solution that could be better if you only want a single component would be to put a zener diode in series with the power, reverse biased so it dropes the zener voltage. This would be more accurate and constant then a diode.