So my dedicated 868Mhz CC1101s transceivers unexpectedly arrived very quickly today, after ordering them just yesterday afternoon 
Want to give a quick update here on what I found so far.
They are a pain to solder up, as the hole spacing is very very tight, but got it nicely all hooked up in the end.
Before actually changing the frequency to 868Mhz I found that the 868Mhz CC1101s still recognised my other 433Mhz commands which I send with a separate OMG gateway. Strangely enough though they were recognised and reported as dedicated rtl_433 protocols/devices, e.g.
{“model”:“Smoke-GS558”,“id”:16723,“unit”:0,“learn”:0,“code”:“28a8a0”,“protocol”:“Wireless Smoke and Heat Detector GS 558”,“rssi”:-64,“duration”:1506110}
{“model”:“Akhan-100F14”,“id”:23756,“data”:“0x4 (Mute)”,“protocol”:“Akhan 100F14 remote keyless entry”,“rssi”:-64,“duration”:1450238}
all this while my 433Mhz RF OMG gateways was just switching some generic power outlets ON and OFF!?!
Maybe something to look into for others already using a 433 RF gateway in addition to a CC1101 rtl_433 one. Or it could be an issue with receiving 433Mhz with the dedicated 8686Mhz CC1101.
After setting the desired frequency with
‘-DCC1101_FREQUENCY=868.30’
and fiddling with the RF_RECEIVER_GPIO a bit, since I’m using an ESP32-WROOM-32D, all seems to be nicely up and running with 868Mhz.
The only 868Mhz devices I have so far are all my Homematic sensors and actuators. There are many of them in the house, and the gateway constantly reports them now, from the outdoor temperature and humidity sensor to the washer and dryer power sensors in the cellar (double checked by watching my openHAB log and the gateway output at the same time), albeit with them being unknown obviously
{“model”:“unknown”,“protocol”:“signal parsing failed”,“duration”:101819,“signalRssi”:-56,“pulses”:221,“train”:1,“messageCount”:253,“_enabledReceiver”:1,“receiveMode”:0,“currentRssi”:-89,“minimumRssi”:-82}
During serial monitoring it also nicely shows how @NorthernMan54’s protocol list tries to possibily match these with certain protocols, including the failing messages.
…
rtl_433_ESP(7): demod(6) - Fine Offset WH1050 Weather Station
rtl_433_ESP(7): demod(6) - Fine Offset Electronics WH1080/WH3080 Weather Station
rtl_433_ESP(7): demod(5) - FT-004-B Temperature Sensor
rtl_433_ESP(7): demod(6) - Generic wireless motion sensor
rtl_433_ESP(7): demod(6) - Generic Remote SC226x EV1527
rtl_433_ESP(7): demod(5) - Generic temperature sensor 1
rtl_433_ESP(7): demod(5) - Globaltronics QUIGG GT-TMBBQ-05
gt_tmbbq05_decode: Possible Quigg BBQ: bitbuffer:: Number of rows: 1
[00] {110} 00 00 00 00 00 00 00 00 00 00 00 00 00 00
…
rtl_433_ESP(7): demod(6) - Philips outdoor temperature sensor (type AJ3650)
philips_aj3650_decode: wrong number of bits (110)
…
These are my findings so far. Not sure if I’ll jump straight into getting the weather station I’m interested in, but will probably read up a bit more on rtl_433 compatible devices first, and on a weather station forum, on how others are setting up and using just the sensors, without the need for additional station receivers/displays.
BTW @NorthernMan54 - is there an easy way to see which of the Enabled Device Decoders are 433Mhz and which are 868Mhz devices, short of googling each of them? 
It would be great if others can share their experience with 868Mhz support on OpenMQTTGateway in this thread.