Using AutoMapper with F#

I'm trying to use AutoMapper from F#, but I'm having trouble setting it up due to AutoMapper's heavy use of LINQ Expressions.

Specifically, the AutoMapper type IMappingExpression<'source, 'dest> has a method with this signature:

ForMember(destMember: Expression<Func<'dest, obj>>, memberOpts: Action<IMemberConfigurationExpression<'source>>)

This is typically used in C# like this:

Mapper.CreateMap<Post, PostsViewModel.PostSummary>()
    .ForMember(x => x.Slug, o => o.MapFrom(m => SlugConverter.TitleToSlug(m.Title)))
    .ForMember(x => x.Author, o => o.Ignore())
    .ForMember(x => x.PublishedAt, o => o.MapFrom(m => m.PublishAt));

I made an F# wrapper that arranges things so that type inference can work. This wrapper allows me to translate the C# example above into something like this:

Mapper.CreateMap<Post, Posts.PostSummary>()
|> mapMember <@ fun x -> x.Slug @> <@ fun m -> SlugConverter.TitleToSlug(m.Title) @>
|> ignoreMember <@ fun x -> x.Author @>
|> mapMember <@ fun x -> x.PublishedAt @> <@ fun m -> m.PublishAt @>
|> ignore

This code compiles, and it seems pretty clean as far as syntax and usage. However, at runtime AutoMapper tells me this:

AutoMapper.AutoMapperConfigurationException: Custom configuration for members is only supported for top-level individual members on a type.

I presume this is caused by the fact that I have to convert Expr<'a -> 'b> into Expression<Func<'a, obj>>. I convert the 'b to obj with a cast, which means my lambda expression is no longer simply a property access. I get