naturally

yeah I guess in these cases you have to do it yourself, would be an interesting idea to generalize,
many things come into mind, e.g. an option to parse with a callback function that adds things to the meta Map in the AST, or an AST walker that keeps access to the parent nodes, …

Unfortunately no time at all.

This should not be too difficult

import Earmark.Transform
iex(11)> {:ok, ast,_} = EarmarkParser.as_ast(" #x")
{:ok, [{"p", [], [" #x"], %{}}], []}
iex(12)> transform = fn
...(12)> x when is_tuple(x) -> x
...(12)> _ -> [ " ", {"a", [{"href", "whatever"}], ["x"], %{}}] end # need to scan the parameter for hashtags and create a structure like the one I just put here
#Function<44.65746770/1 in :erl_eval.expr/5>
iex(13)> ast1 =map_ast(ast, transform)
[{"p", [], [[" ", {"a", [{"href", "whatever"}], ["x"], %{}}]], %{}}]
iex(14)> transform(ast1, [])
"<p>\n <a href=\"whatever\">x</a></p>\n"

HTH

I just realized with the release of Earmark 1.4.25 there are two ways to achieve your goals with Earmarks tools at hand

even before 1.4.25 you could have used map_ast_with to keep track of the condition if to replace hashtags or not in the accumulator

and now from 1.4.25 you can also, which IMHO is more elegant, change the mapper function dynamically when traversing with map_ast by not returning a quadruple but a tuple with the new function and a quadruple

here
you can find an example of how to do that