I'm not very familiar with power factor correction controllers and haven't had much luck in finding a controller that accepts a control signal, can you point me in the right direction? Its worth considering if it will also allow for fine-grained control 
I'm no PFC expert either but I did look into the early PFC control chips very closely (probably 15-20 years ago, when I was needing to build a large PFC'd battery charger for an off-grid set-up to pull more power from the generator than the stock 100A charger could) when they started to arrive on the market due to increasing requirements for PFC in more and more power supplies owing to the to various government regulations to help the power companies clean up their modern, messy power.

Today, most typical PFC chips tend to be little 6-pin "do it all automatically" affairs for generic consumer power supplies rather than the uber-adjustable early stuff, though pretty much
any a modern PFC chip could be used if you simply vary the divider voltage coming from your output (to the load) to the voltage sense input. You could control it that way if needed by using some external circuitry to "fool" the
VSENSE pin into thinking the output voltage is higher than it is.
However, while I don't recall any of the early ones that I saw that specifically had a "control" pin, some chips do still give you various kinds of additional inputs one way or another so you don't have to mess with the
VSENSE divider itself from your output voltage.
For example, the TI UCC28180 would probably work in your application. It has an integrated 1.5A source / 2A sink capable MOSFET driver built in, so can probably drive MOSFETs large enough for a few kW regulator circuit, plus it has access to the output side of the sense amplifier via the
VCOMP pin which can be used to do soft-start, modify your output control loop sensed-voltage, pull the whole PFC regulator into standby, etc. I would imagine that with simple external circuitry that could include input from your "excess solar power calculator" circuit.
http://www.ti.com/lit/ds/symlink/ucc28180.pdfI don't see why something like that wouldn't work. I'm not sure it is the easiest or best way, it is just something to consider, since what you're trying to achieve is essentially what PFC regulator does in the first place anyway. There must be
Application Notes around for various PFC set-ups... Anyone "in-the-know" want to chime in?
