Use the Objc runtime and va_args to determine and use method parameters at runtime.

swizzle_any_method.m

#import "TypeEncodings.h"

@interface Foo : NSObject
- (char)bar:(NSString *)bar baz:(int)b;
@end
@implementation Foo

- (char)bar:(NSString *)bar baz:(int)b {
    printf("%s\n", bar.UTF8String);
    return '#';
}
@end

int main() {
    // NSInvocation hack to invoke the desired IMP
    void (*invokeWithIMP)(id, SEL, IMP) = (void *)[NSInvocation instanceMethodForSelector:NSSelectorFromString(@"invokeUsingIMP:")];
    
    @autoreleasepool {
        Method method = Method(@selector(bar:baz:), [Foo class]);
        IMP orig = originalIMP(method);
        NSInvocation *invocation = [NSInvocation invocationWithMethodSignature:NSMethodSignatureOfMethod(method)];
        TypeEncoding returnType = invocation.methodSignature.methodReturnType[0];
        
        IMP imp = imp_implementationWithBlock(^void *(id receiver, ...) {
            // Give invocation the original arguments
            va_list args;
            va_start(args, receiver);
            
            for (NSInteger i = 2; i < invocation.methodSignature.numberOfArguments; i++) {
                const char *typec = [invocation.methodSignature getArgumentTypeAtIndex:i];
                TypeEncoding type = typec[0];
                
                switch (type) {
                    case TypeEncodingInt: {
                        int baz = va_arg(args, int);
                        [invocation setArgument:&baz atIndex:i];
                        printf("arg2: %d\n", baz);
                        break;
                    }
                    case TypeEncodingObjcObject: {
                        id bar = va_arg(args, id);
                        [invocation setArgument:&bar atIndex:i];
                        printf("%s", [NSString stringWithFormat:@"arg1: %@\n", bar].UTF8String);
                        break;
                    }
                    
                    default: {
                        printf("Some other type we don't care about: %c\n", (char)type);
                        exit(1);
                    }
                }
            }
            
            // Invoke method with original IMP
            invocation.target = receiver;
            invokeWithIMP(invocation, 0, orig);
        
            // We know the return length is greater than zero for this
            // case, but if you wanted to dynamically return a value
            // or not based on the return type, this is how to do it
            if (invocation.methodSignature.methodReturnLength) {
                void *ret = NULL;
                [invocation getReturnValue:&ret];
                return ret;
            } else {
                return NULL;
            }
        });
        
        // Make Foo use our new method implementation
        swizzle(method, imp);
        
        Foo *f = [Foo new];
        char ret = [f bar:@"hello" baz:5];
        printf("Returned: %c\n", ret);
    }
}

TypeEncodings.h

#import <Foundation/Foundation.h>
#import <objc/runtime.h>


#define Method(sel, cls) class_getInstanceMethod(cls, sel)
#define originalIMP(m) method_getImplementation(m)
#define NSMethodSignatureOfMethod(m) [NSMethodSignature signatureWithObjCTypes:method_getTypeEncoding(m)]
#define swizzle(m, imp) method_setImplementation(m, imp)

typedef NS_ENUM(NSUInteger, TypeEncoding)
{
    TypeEncodingUnknown          = '?',
    TypeEncodingChar             = 'c',
    TypeEncodingInt              = 'i',
    TypeEncodingShort            = 's',
    TypeEncodingLong             = 'l',
    TypeEncodingLongLong         = 'q',
    TypeEncodingUnsignedChar     = 'C',
    TypeEncodingUnsignedInt      = 'I',
    TypeEncodingUnsignedShort    = 'S',
    TypeEncodingUnsignedLong     = 'L',
    TypeEncodingUnsignedLongLong = 'Q',
    TypeEncodingFloat            = 'f',
    TypeEncodingDouble           = 'd',
    TypeEncodingCBool            = 'B',
    TypeEncodingVoid             = 'v',
    TypeEncodingCString          = '*',
    TypeEncodingObjcObject       = '@',
    TypeEncodingObjcClass        = '#',
    TypeEncodingSelector         = ':',
    TypeEncodingArray            = '[',
    TypeEncodingStruct           = '{',
    TypeEncodingUnion            = '(',
    TypeEncodingBitField         = 'b',
    TypeEncodingPointer          = '^'
};