Solar water heaters interface well with radiant floor heating systems, or with hot water baseboards and radiators. The question is: is it worthwhile, or advantageous? The answer, for energy-efficient homes is... No.
Connecting a solar water heater to a radiant floor heating
Radiant floor heating systems work at relatively low water temperatures, and solar water heaters haven't any trouble with supplying hot water at such temperatures... under normal sunlight conditions.
The problem is at nighttime or during cloudy weather, in cold climates, where radiant floor heating makes most sense.
In these last climates you will need a boiler or a condensing water heater, or other backup water heating heating system, which is technically easy to implement, but makes the whole system a lot more expensive...
Solar Water Heating & Baseboards and Radiators
Interfacing solar water heaters with baseboards or radiators is not difficult either. Hot water radiators/baseboards require higher temperatures than radiant floor heating, and also a boiler or a similar water heating system, but that too is technically easy to achieve.
Worthwhile and advantageous?
But the initial question remains: is a hybrid solar-hydronic heating system advantageous? The short answer, from energy-efficient standpoint, is NO.
And the reasons aren't hard to devise: solar water heaters are expensive in climates with freezing temperatures (see: Solar Water Heaters Prices and Paybacks); and it doesn't make sense to install a large solar water system for space heating purposes, in these climates.
The combination makes a lot more sense in climates where solar water heating is cost-competitive: some moderate and hot climates. But... central hydronic system are mostly useless in these climates, at least in energy efficient homes.
Though radiant floor heating and baseboards provide a very comfortable type of heat, the whole system is expensive and not really advantageous in homes with high levels of insulation and air-sealing, and high-performance windows. In other words: they are too costly and innefective in energy-efficient homes (see: Radiant Floor Heating Guide and Hydronic/Hot Water Space Heating).
The bottom line
Solar-hydronic space heating systems are technically easy to achieve; but they do not make sense in energy-efficient homes or from an environmental standpoint.