HC-49 Crystals are very delicate and will not stand up to the large currents forced through them by many classical oscillator circuits.  But they are readily available and are inexpensive compared to the old FT-243 crystals that can handle large currents.

This W1TS Vintage Transmitter Design uses the HT-49 in its 6C4 oscillator circuit.  Appendix I of his design explains how to use HT-49’s.  The output stage of this design could be used as a buffer/driver for a higher-wattage final such as an 807, 6146B, etc., should more power be desired.  Radiation of harmonics would be at a minimum.

Another article addresses the use of HC-49’s in oscillators: David Newkirk Writings_ Enjoying Radio_ HC-49-Crystal-Safe Vacuum Tube Transmitter Designs 

These two articles should provide enough information for the design and use of inexpensive HC-49 crystals in QRP (or higher power) transmitters.

Since they are relatively inexpensive, it would be easy to install several switch-selected ones in an oscillator, providing even more flexibility.