This is as far as I got - so close, and yet so far:
from com.inductiveautomation.ignition.common.expressions.parsing import ELParserHarness
ELParserHarness().parse("1+1", None).execute()
The trick is acquiring an ‘ExpressionParseContext’ for the second argument to parse() - which I’m fairly certain requires a custom module. Without that, expression functions don’t work.
Also - a disclaimer: This is not officially supported or encouraged.