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MwRSF has previously answered a similar question were a water drainage structure was obstructing the placement of a post.  For w-beam systems (MGS) not in a transition regions, it was recommended to simply omit a single post at the obstruction location.  The system would act as a 12.5 ft long span system - recall, MGS long-span was crash tested to 25 ft span without the need for nesting the rail. 

If a drainage structure is obstructing more than one post installation, then bolting the posts to the top of the intake would be a better solution.

When you say "simply omit a post," I assume you mean omit a post, but install the CRT posts upstream and downstream of omitted post, correct?  Or is simply omitting a post where there is not a culvert opening/dropoff allowable?

If this particular run of guardrail was installed flush with a curb, would that change the recommendation at all?

And finally, should half-post spacing be utilized when bolting to the top of an intake - especially where two or more consecutive posts cannot be installed?

NOTE: feel free to call me if you'd like to discuss this.

Yes, we recommend using CRT posts on the upstream and downstream ends of the region where posts will be omitted. The MGS Long Span system was designed with 3 CRT posts on both the US and DS ends and can be used to span lengths up to 25 ft (the equivalent of 3 missing posts).  Although the required number of CRT posts may go down when omitting only 1 or 2 posts, until we have testing to illustrate the crashworthyness of the modified system, MwRSF will continue to recommend using 3 CRTs on each side to prevent pocketing and snag.

I do not anticipate a major issue with the guardrail being installed flush with a curb.  The MGS was successfully tested with a 6" offset from a curb.  Installing the system flush with the curb should ensure the vehicle and guardrail interlock, even if the system is missing a post or two.

The MGS long span system can be installed with span lengths of up to 25 ft, same as 3 omitted posts.  If the drainage structure you are installing guardrail over requires more than 3 omitted posts, the 1/2 post spacing guardrail system designed for culverts would be a possible solution.  However, you would need to be carefull in attaching the posts to ensure (1) the drainage structure can handle the post-anchor loads, and (2) the posts are installed with the correct length - recall these posts extended 9" below groundline before attaching to the top of the culvert slab.