This is indeed currently not possible in the main Lansweeper search since it entails a full text search. Expanding what can be searched has already been added to the wishlist. This will mean a rework of how the search currently works, so not something for the near future.
You can currently achieve the requested by creating a custom report like the example below. This will allow you to go to the report and then use the comments filter to search for a concrete comment.
Select Top 1000000 tblAssets.AssetID,
tblAssets.AssetName,
tsysAssetTypes.AssetTypeIcon10 As icon,
tblAssetCustom.Comments
From tblAssets
Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
Inner Join tsysAssetTypes On tsysAssetTypes.AssetType = tblAssets.Assettype
Where tblAssetCustom.State = 1
