Alias a JSON-LD keyword and simultaneously provide a @context for it

I have the following JSON document (based on GitHub API output):

{
  "@id": "https://github.com/octadocs/octadocs",
  "license": {
    "key": "mit",
    "name": "MIT License",
    "spdx_id": "MIT",
    "url": "https://api.github.com/licenses/mit",
    "node_id": "MDc6TGljZW5zZTEz"
  }
}

Using JSON-LD, I'd like to retrieve the following triple from this:

<https://github.com/octadocs/octadocs>
    <https://octadocs.io/github/license>
    <https://spdx.org/licenses/MIT> .

• I can interpret license as https://octadocs.io/github/license with @vocab (I would like to use it globally actually, for all properties);
• I can specify @type for spdx_id to make its value an IRI;
• and I finally can define a @context with a @base for it, to convert MIT string into an spdx.org reference.

Context:

{
  "@base": "https://octadocs.io/github/",
  "@vocab": "https://octadocs.io/github/",
  "spdx_id": {
    "@type": "@id",
    "@context": {
      "@base": "https://spdx.org/licenses/"
    }
  }
}

See a demonstration in JSON-LD playground.

But, this creates a slightly different structure than the desired one:

<https://github.com/octadocs/octadocs> <https://octadocs.io/github/license> _:b0 .
_:b0 <https://octadocs.io/github/spdx_id> <https://spdx.org/licenses/MIT> .

I'd like to avoid the blank node.

This can be achieved by JSON-LD keyword aliasing:

{
  ...,
  "spdx_id": "@id"
}

but how to simultaneously

• alias the property,
• and define things like @type and @context for it?