| Welcome to JShell -- Version 19
| For an introduction type: /help intro

jshell> import java.lang.invoke.*;

jshell> class C { static void test() throws Throwable {
   ...> MethodHandles.Lookup lookup = MethodHandles.lookup();
   ...> MethodHandle mh = lookup.findSpecial(Object.class, "toString", MethodType.methodType(String.class), C.class);
   ...> System.out.println(mh.invoke(new C()));
   ...> }}
| created class C

jshell> C.test()
REPL.$JShell$17$C@b1bc7ed

jshell> interface I { static void test() throws Throwable {
   ...> MethodHandles.Lookup lookup = MethodHandles.lookup();
   ...> MethodHandle mh = lookup.findSpecial(Object.class, "toString", MethodType.methodType(String.class), I.class);
   ...> System.out.println(mh.invoke(new I(){}));
   ...> }}
| created interface I

jshell> I.test()
| Exception java.lang.IllegalArgumentException: nothing to resolve
| at MethodHandleNatives.resolve (Native Method)
| at MemberName$Factory.resolve (MemberName.java:1085)
| at MemberName$Factory.resolveOrNull (MemberName.java:1129)
| at MethodHandles$Lookup.getDirectMethodCommon (MethodHandles.java:4010)
| at MethodHandles$Lookup.getDirectMethod (MethodHandles.java:3963)
| at MethodHandles$Lookup.findSpecial (MethodHandles.java:3012)
| at I.test (#4:3)
| at (#5:1)

There's no particular reason that an interface shouldn't be able to make the same 'findSpecial' request as a class. I believe this 'findSpecial' call should succeed.