Question is nicely formatted here, but a copy and paste:-

Given this as input.md:-

```markdown
---
title: Strings Are Evil
subtitle: Reducing memory allocations from 7.5GB to 32KB
header-includes: |
    <meta name="og:type"                property="og:type" 
content="website">

    <meta name="title"                  property="title" 
content="{{title}}" />
    <meta name="description"            property="description" 
content="{{subtitle}}" />
    
    <meta name="og:title"               property="og:title" 
content="{{title}}" />
    <meta name="og:description"         property="og:description" 
content="{{subtitle}}" />

    <meta name="twitter:title"          property="twitter:title" 
content="{{title}}" />
    <meta name="twitter:description"    property="twitter:description" 
content="{{subtitle}}" />
---
rest of the blog
```

Produce this output:-

```html
<meta name="og:type"                property="og:type" content="website">
<meta name="title"                  property="title" content="Strings Are 
Evil" />
<meta name="description"            property="description" 
content="Reducing memory allocations from 7.5GB to 32KB" />
<meta name="og:title"               property="og:title" content="Strings 
Are Evil" />
<meta name="og:description"         property="og:description" 
content="Reducing memory allocations from 7.5GB to 32KB" />
<meta name="twitter:title"          property="twitter:title" 
content="Strings Are Evil" />
<meta name="twitter:description"    property="twitter:description" 
content="Reducing memory allocations from 7.5GB to 32KB" />
```

I did try 
https://pandoc.org/lua-filters.html#replacing-placeholders-with-their-metadata-value 
and played about with 
https://github.com/jgm/pandoc/issues/1950#issuecomment-74613325 but I could 
never get it to work inside the markdown meta block.

I've tried various incarnations of `{.thing}` too.

Cheers,
Indy

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