Personally, I prefer to avoid parking analog locos on digital layouts. Since the motor is constantly pulsed in both directions and the motor doesn't spin to cool itself, there is the potential for overheating. Even when the motor is running, it still receives pulses with the opposite polarity.
I believe Bob Grosh or another member of the forum reported motor damage from zero stretching.
Short periods of use are probably okay, but if you are planning to run the loco often on the digital layout you would be better served to add a decoder.
Locos with Faulhaber (coreless) motors probably won't like zero-stretching operation. Since a coreless motor lacks the heatsinking benefit of an iron core, the motor will overheat more easily.
In general, LGB analog sound systems can be used on MTS tracks without fear of damage. Since the track voltage is typically in the range of 22-24V, there is no fear of overvoltage.
I do not know about LGB or Massoth systems. However, the reverse may be a problem. You are worried about damaging a loco by running it on DCC without a decoder. Well, a lot of people running DCC turn off the analog feature as it interferes with some of the DCC capabilities.
There are many threads about locos with decoders not working properly on a layout that allows the analog feature. So many of the people with larger layouts are disabling it in the command stations and also in the decoders. It was a nice feature when people were getting started so you could run one loco on DCC and one on analog at the same time without buying a second decoder and spending the time installing it. However, as more locos decoders were installed it became of much less worth to people.
As far as I have read, almost no non-DCC engines are harmed by running on the analog feature. This is a consensus, some people have reported problems but most have not seen them. It is almost unanimously agreed that running an analog loco for a short time will not harm it.
You can not run a "core-less motor" on DCC unless it has a decoder..
...and the DCC decoder used with a coreless motor should also be capable of supplying a high PWM frequency. A minimum PWM frequency of 32kHz is preferred. There are several decoders on the market capable of meeting this requirement.