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