Empirically determined the best setup for motion vectors in OpenGL, though it still doesn't make a whole lot of sense. Added opt-in debug symbols line to the upscale shaders to make analysis easier.

Packages/com.unity.postprocessing@3.2.2/PostProcessing/Runtime/Effects/Upscaling/SGSR2/Shaders/2_pass_cs/sgsr2_upscale.compute

@@ -1,4 +1,5 @@
 #pragma kernel CS
+//#pragma enable_d3d11_debug_symbols
 #include "../sgsr2_birp.hlsl"
 #include "../sgsr2_common.hlsl"
 

Packages/com.unity.postprocessing@3.2.2/PostProcessing/Runtime/Effects/Upscaling/SGSR2/Shaders/3_pass_cs/sgsr2_upscale.compute

@@ -1,4 +1,5 @@
 #pragma kernel CS
+//#pragma enable_d3d11_debug_symbols
 #include "../sgsr2_birp.hlsl"
 #include "../sgsr2_common.hlsl"
 

Packages/com.unity.postprocessing@3.2.2/PostProcessing/Runtime/Effects/Upscaling/SGSR2/Shaders/sgsr2_birp.hlsl

@@ -22,5 +22,9 @@ SamplerState s_LinearClamp  : register(s1);
 inline float2 decodeVelocityFromTexture(float2 ev)
 {
     // Nothing much to do, motion vectors are not encoded
+#if UNITY_UV_STARTS_AT_TOP
     return -ev;
+#else
+    return ev;
+#endif
 }