If the power requirements/expectations are low from the planar device it will work as an isolator but, as has been mentioned in Spehro's answer, the high inductance required to connect it directly to 230V AC, means the number of turns are unfeasible. You need high primary inductance to avoid core saturation.
Consider a standard mains transformer with a primary magnetization inductance of 10 henries has an impedance of 3142 ohms at 50Hz. To get this inductance it may have 1,000 turns ball-park. The primary mag current is 230V/3142 = 73 mA.
Ampere turns are therefore 73.
If the transformer only had 500 turns the inductance would be one-quarter (inductance in a coil like this is proportional to turns-squared) and the reactance would be 786 ohms. Now the current is 292 mA and, more importantly the ampere turns is 146.
Ampere turns create saturation and, it can be seen (hopefully) that fewer turns are going to rapidly lead to disaster in terms of performance. One or two folk may be murmering that "surely, the ampere-turns are far more when the transformer is under load and that the above is a pointless calculation" (or words to that effect).
No, saturation occurs at its maximum when a transformer is unloaded. Ampere turns due to load in the primary are cancelled by ampere turns in the secondary.
Anyway I'm going off at a bit of a tangent. What I'm trying to say is that you can use a planar transformer at 50Hz but connected to the AC via a capacitor. You can't get much power from it but if you can get enough turns on the primary to (say) work at 50V AC you can engineer a solution where the cap drops 180V.
There are of course load dependancies with this arrangement but, if the output is regulated then you can get a few watts from the device easily.
Who would do such a thing? I really don't know but it is possible and, maybe there is some application out there crying for this as a solution.