Given a complex time-domain signal is it always implied that this is a QAM-demodulated form of a real time domain signal?
No, we generally use complex baseband to describe all kinds of signals, or even passband systems (e.g. frequency-selective or time-variant channels). So, any bandlimited signal has an equivalent complex baseband, and it's not implied this has anything to do with QAM or even digital communication.
QAM is just one example of something that is really easy to write down in complex baseband.
Are there any other commonly used ways of converting a real time domain signal to a complex time domain signal that don't imply QAM?
I don't even know how QAM comes into this – you transform any real-valued bandpass signal to complex baseband through multiplication with its (complex) carrier and low-pass filtering. QAM really has nothing to do with it.