How to mark a Java method as "must use result" for static analysis purposes?

There is totally a standard annotation for this, and it is @CheckReturnValue. FindBugs has it; see e.g. here.

Guava uses it internally -- e.g. in the configuration methods for Splitter -- from JSR 305.

Use

import javax.annotation.CheckReturnValue;
.
.
.
@CheckReturnValue

Some good examples of @CheckReturnValue are available on the Google error-prone project wiki. (If you like static analysis tools such as FindBugs, you should definitely check out error-prone; it works on the source/AST rather than the bytecode, which makes it complementary to tools such as FindBugs.)