OmniFocus (and custom url scheme) Link in Notion work around options
Use a custom HTML file (automated)
This jsfiddle will generate and download an html file that redirects to pasted link that can be embed into a Notion Page:
Or JSFiddle (if you want to see how it works)
On Notion, choose the embed block > select the upload tab > and choose the html file
Use custom HTML (manual)
This is how to manually create html file that in automatically created above
Create an html file with the following contents (replacing my omnifocus url with yours) (using your favorite plain text editor and saving it as a .html file).
<head>
<meta http-equiv="refresh" content="0; URL=omnifocus:///task/pg3Evi4aepc" />
</head>