Yes. HDR techniques can be used with long exposures just as with short exposures. The main issue is not shutter length itself, but motion: waves, clouds, foliage, or changing water blur differently between frames, which can confuse automatic HDR merging and create artifacts.

Common approaches:

• Shoot multiple exposures and merge them as HDR.
• Manually blend exposures with masks, which is often preferred in landscapes when movement makes automatic HDR unreliable.
• In some cases, a single RAW file may contain enough dynamic range to recover sky and foreground detail and then be tone-mapped, giving an HDR-like result without true multi-exposure HDR.

To create different exposures, you can vary shutter speed, ISO, aperture, or use ND filters. Aperture changes depth of field, so it’s usually the least desirable variable for HDR bracketing. If using shutter-speed bracketing, taking the shortest exposure first can help reduce motion-related problems.

So yes: long exposure plus HDR or exposure blending is absolutely possible. The practical limitation is subject movement, and manual masking is often the cleanest solution when automatic merging struggles.