I wonder if a small speaker element would do the trick. The electro-mechanical kind [coil of wire around a permanent magnet]. They are, essentially, short throw linear actuators, and I’ve seen ones that are small and quite flat. So, I guess it would be a matter of whether or not they have enough mass & and enough low frequency response, to create the required type, and level of sensation.
Thinking out loud: First, try it as a normal speaker. But, if that doesn’t produce the desired effect, then perhaps if mounted from the diaphragm [probably a mylar-ish diaphragm is indicated], the housing/magnet would, having the greatest freedom of movement, be more likely to produce greater action, and thus, having more mass, be more likely to create the desired sensation. So, perhaps the diaphragm is, somehow, extended out to contact the skin, or some sort of band that is wrapped around a limb. Perhaps a ring, made from slicing a tube, could be glued to the diaphragm. Then, the other side of the ring made to contact the skin, or the mounting apparatus.
BUT, encumbering the movement of the diaphragm might lower the impedance of the speaker to the point where normal drive would burn it out. So, some experimentation would, probably, be needed. Also, if this is ultimately, to go into production, then realize that using a device outside of it’s published parameters, will, likely, void any support from the manufacturer – unless you actually contact the manufacturer and work out some sort of arrangement.