Jakub owns a french STE without a TV modulator. I'll refer to it as SECAM STE (but I do not know if it is really SECAM related or just Tramiels having a stock of clock oscillators to use). It has a bit different main clock which causes some timing sensitive effects to fail. Jakub did some measurements and come to following measurement:
#define ATARI_STE_SECAM_MCLK 31922046 /* CPU_Freq = 7.9805115 MHz */
Which he used to test modified Hatari build against his latest demo
https://demozoo.org/productions/355416/ that has a number of overscan/colorfull effects and supports STE SECAM timings.
Attached are test programs written by Jakub.
Also I and Jakub think it would be nice to have a switch in Hatari to be able to switch between PAL/NTSC/SECAM models.