Can low energy photons in equilibrium with a black body absorber create a high temperature in the absorber? With enough flux can low energy photons warm a black body to any temperature?
eg a solar furnace can approach the surface temperature of the sun, but could it in theory go to any temperature? If all the photons are absorbed then in equilibrium the radiated power and the temperature can be raised to any level. But maybe there is a limit on how much can be absorbed?
 A: 
With enough flux can low energy photons warm a black body to any temperature?

Yes. A black body absorber can absorb all of the incident radiation. If that amount of energy exceeds the amount that it can radiate at its current temperature then its temperature will increase. However, this does not imply the answer you expect in the next part of your question.

a solar furnace can approach the surface temperature of the sun, but could it in theory go to any temperature?

No, a solar furnace can only reach the temperature of the sun, not exceed it.
This seems like a contradiction, but it is not. The issue is the conservation of etendue, which is an important but little known concept in geometric optics.
The conservation of etendue states that in an ideal passive optical system (ideal mirrors and lenses, no scattering) the product of the area and the solid angle is constant. This is just a geometrical fact of ray optics. The solid angle is limited to $2\pi$ so the maximum etendue of a system is $2\pi A$ where $A$ is the surface area of the smaller object. That means that it is fundamentally impossible to capture all of the thermal radiation from the larger object and direct it to the smaller object. The light from the larger object simply cannot be focused that small.
Planck’s law gives the thermal power radiated at each wavelength per unit area and unit solid angle. In other words, the power is proportional to the etendue and therefore is also fundamentally limited. At a given temperature there is a limit on how much radiative power can be exchanged between two objects. This makes it so that the thermal radiation power will always flow from the colder object to the hotter regardless of how you arrange the mirrors and lenses and regardless of the sizes of the objects.
So a solar furnace cannot exceed the temperature of the sun, but how is that consistent with the earlier “yes” answer? For thermal radiation and passive optics like a solar furnace, the power will always flow from the hotter to the colder object, due to Planck’s law and the conservation of etendue.
But it is possible to use non-thermal radiation, like a laser, and non-passive optics, like a solar panel. You could take solar energy, collect it with a photovoltaic array, then use it to power a laser, which could heat a target to a higher temperature than the sun (and the laser), despite using relatively low energy photons.
