Is there a way to shift light frequency outside of the laser? That could open up more applications.
There are frequency multipliers, optical mixers and optical parametric amplifiers, which can multiply, sum or subtract the frequencies of laser beams.
A well known use is in the green laser pointers, which contain a frequency doubler and an infrared laser.
However any frequency conversion device increases the cost and reduces the energy efficiency.
It is very unlikely that it would ever make sense to multiply the frequency of such a laser with enough stages to reach visible light, because that would increase the cost much above the alternative solutions that exist for tunable lasers in the visible range.
These can still have many useful applications, but holograms are not one of them, because their output will not be visible.
Laser cutters are also another excluded application, because such lasers will have a low electrical energy to light energy conversion efficiency. The reason is that these solid-state free-electron lasers, like also most other laser types except the laser diodes and some of the gas lasers, use another laser as their energy source, so the efficiencies of the 2 lasers are multiplied. Actually the pumping laser needs almost certainly to be pumped itself by a laser diode. Therefore the total efficiency will be the product of the efficiencies of 3 lasers, so it will be low.
More likely applications would be in communications, Lidar, imaging through materials that are opaque for visible light, chemical analysis, maybe influencing certain chemical reactions.
The fact that solid-state free-electron lasers should work only in the far infrared is actually expected, because high frequencies, like those of visible or ultraviolet light require a large energy difference between the energy of the electrons before and after emitting the laser light. When the electrons are free before and after emission, that means that the pumping source must accelerate them, providing the energy difference.
In vacuum, a free electron can be accelerated to any energy, e.g. up to levels enabling the emission of X-rays when the electrons lose the accumulated energy, but in a metal a free electron that is accelerated too much will either collide with the lattice of atoms, losing the energy, or it might even exit the metal and be lost, like in the photoemissive electrodes that were used in the earliest video cameras.