Yes, since v0.6.3.9, GLSL Hacker supports MRT.
There should be a demo in the code sample pack /host_api/MultipleRenderTargets/
To create a render target with two color buffers:
num_color_targets = 2
rt01 = gh_render_target.create_v2(width, height, num_color_targets)
When you need to bind color buffers as textures, just do:
gh_texture.rt_color_bind_v2(rt01, 0, 0) -- Color texture 0 on texture unit 0
gh_texture.rt_color_bind_v2(rt01, 1, 1) -- Color texture 1 on texture unit 1
In the fragment shader, just declare several outputs:
out vec4 FragData;
Let me know if the code sample pack is not up to date.
Hope that helps.