As it often is with SharePoint, your best answer you will find yourself by looking into the inner working of things.
The problem is that the HashTags column is set to Hidden in a Community discussion board. Hidden columns are not indexed by SharePoint search so you cannot find them. Easy solution: Set the column to visible, only that this is a bit more complex as CanToggleHidden is set to false...We will need some ugly reflection for that. The following powershell will set the column to visible and allow search to crawl.
$web = Get-SPWeb http://myweb
$list = $web.GetListsOfType([Microsoft.SharePoint.SPBaseType]::GenericList) | ? {$_.BaseTemplate -eq "DiscussionBoard"}
$fld = $list.Fields["HashTags"]
$hiddenState = $fld.Hidden
Write-Host "Currently 'HashTags' Hidden is set to $hiddenState"
if ($hiddenState) {
$type = $fld.GetType()
$method = $type.GetMethod("SetFieldBoolValue",[System.Reflection.BindingFlags]$([System.Reflection.BindingFlags]::NonPublic -bor [System.Reflection.BindingFlags]::Instance))
$method.Invoke($fld, @("CanToggleHidden",$true))
$fld.Hidden=$false
$fld.Update()
Write-Host "Now field 'HashTags' is set to visible"
}
You will need a full crawl after setting the HashTags field to visible.
Bear in mind that the HashTag profile page on the MySite as well as the Trending hashtags WebPart also on the MySite will still only show Newsfeed Hashtags. The underlying search for both pages is hardcoded to only look for newsfeed content types - as discussion lists have a discussion and a message content type this way you won't find any HashTags for these items.
On a technical note: The search query being used (Microsoft.SharePoint.Portal.WebControl.HashTagProfile.InitTaggedItem) on the HashTagProfile page is "NOT(MicroBlogType:2 OR MicroBlogType:4 OR ContentTypeId:0x012002* OR ContentTypeId:0x0107* OR WebTemplate=COMMUNITY)owstaxIdMetadataAllTagsInfo:#0" - so the code is explicitly excluding community content. Even though all HashTag links in communities point to the FeedRedirector and in turn to the HasTagProfile page.. won't work.
