European appliances in on US Hot-to-Hot supply
From a wiring point of view, you should be safe enough. Unlike in the US, European appliances are all designed to be operated safely in either polarity. European plugs (both grounded and non grounded types) can be inserted in either polarity. The approach taken here, rather than polarising the plugs and outlets, was to ensure that appliances could operate safely in either polarity.
The reasoning behind this is that in the 1930s, when these standards were fixed, there were some supplies on hot-to-hot and others on hot-to-neutral 220V 50Hz. So, polarity was really a non-issue.
When a standard Euro CEE 7/7 plug is used in a CEE 7/4 socket (German style outlet) it's polarity is random.
When the same plug is used in a CEE 7/5 socket outlet (French style) it's fully polarised.
The UK and Ireland use a different system, where we only have 3-pin plugs. They can only be inserted one way, so they're always polarised.
The reality is it doesn't really matter either way, so if you're using a European appliance on a US supply, you should be ok from the point of view of polarity.
The risks are the fact that it's a 60Hz supply and also that the voltage may be too high. Your supplies are officially 240V +/- ??%. They may be slightly too high voltage for some sensitive 230V appliances and may burn things out.
I'd suggest using a multimeter across the terminals of a US 240V outlet and checking the actual voltage supplied before connecting a Euro appliance!!
Here's a European CEE 7/7 16A grounded plug (Grounding is via the strips at top/bottom (German style) or via a pin which mates with the hole on the front (French style))
