if a plan participant is over age 59 1/2 and is taking an in-service withdrawal from a 401(k) plan of her elective deferrals, what is the mandatory fed tax withholding? 20%? any way around this?

Amounts which are eligible to be rolled over to an IRA (or plan of another employer), if paid in cash, are automatically subject to 20% withholding at the federal level. State laws vary, and some do require withholding. The "way around this" is to actually do the rollover, rather than take it in cash.

See IRS Publication 575 page 8.

If the in-service is taken as a hardship distribution, it is not rollover eligible and therefore not subjected to the 20% w/h rules