The Yaesu VX-5R had/has 50-54 MHz FM, 5 watts. Nice compact little radio!
The VX-7 and VX-8 can also transmit 5 watts FM on 6 m, and 1 watt AM. None of the 3 will transmit SSB.
It had a little extension of the rubber duck to make it work at 50 MHz, though I doubt it was very efficient.
The whip is about 25 cm long, so I'll consider a 0.5 m dipole. Radiation resistance the dipole is 1.4 $\Omega$. Reactance is about -1500 ohms, resonating this with a coil will introduce the rest of the 50 $\Omega$ resistance, probably giving a reasonable match. Efficiency is thus about -15 dB.
When talking to someone with a full-size dipole, like a repeater, the gain of the 6 m antenna will be partially compensated for by the reduced path loss. The path loss is 6 dB smaller at 6 m, compared to 2 m. Assuming the gain at 2 m is -3 dBi, and the gain at 6 m is -15 dBi, at 6 m the handheld will deliver only 6 dB less signal to the repeater.