The talented community member @Retrostuff_org (so not Adam Zey, aka @Guspaz who owns webshop retrostuff.ca) has made good progress regarding Region mod on the Philips CD-i players.
While most of the models can be simply modded for 50/60Hz, changing properly from PAL to NTSC or vice versa is tricky. In his article, the person behind RetroStuff.org explains how to achieve so by using a DFO (Dual Frequency Oscillator).
On top of that, he has written a complete guide on which mod can done on which model regarding 50/60Hz, PAL/NTSC, RGB and NVRAM.
I strongly recommend you follow him on Twitter for his dedicated work on rare video game hardware :-).