Rule 105 of Reg M disallowing shorting only applies to underwriters and instututional investors. If you as an individual layinvestor wish to loan me, another indivudual layinvestor, 100 shares of an IPO to short during its first day of trading, there is no rule prohibiting this.
I'm not sure how practical this is, though, nor how major unaffiliated brokarage houses might be affected under different regulations. (Anyone know or try this?)