You should definitely check the handlebars documentation first.
You can pass a whole Map into your HTML and then iterate and use your helper (aCustomHelper). The '@key' keyword refers to the key of the map and the 'this' keyword refers to the value of the map (example-map-object).
{{#each example-map-object}}
<tr>
<td>{{{this}}}</td>
<td>{{{@key}}}</td>
<td>{{{aCustomHelper @key this}}}</td>
</tr>
{{/each}}
In the example below, you can see how you can access a Set object by key and value:
<select>
{{#each example-set-object}}
<option value="{{@value}}">{{@key}}</option>
{{/each}}
</select>
In the example below, you can see how you can access a Map by it's key values, which is very handy indeed:
{{#each example-map-object}}
<tr>
<td>{{{this.id}}}</td>
<td>{{{this.customerName}}}</td>
<td>{{{this.date}}}</td>
</tr>
{{/each}}
So the answer to your question is that YES, you can most certainly do what you want and pass the whole map as a parameter to your helper method.
{{yourHelper example-map-object}}