So I tried and I got a very basic UDF working.
mysql> SELECT udf_fileexists_go("/etc/hosts");
+---------------------------------+
| udf_fileexists_go("/etc/hosts") |
+---------------------------------+
| 1 |
+---------------------------------+
1 row in set (0.00 sec)
mysql> SELECT udf_fileexists_go("/nonexistend");
+-----------------------------------+
| udf_fileexists_go("/nonexistend") |
+-----------------------------------+
| 0 |
+-----------------------------------+
1 row in set (0.00 sec)
This is nowhere near production quality, so be careful.
The code is here:https://github.com/dveeden/udf_fileexists_go/blob/master/udf_fileexists_go.go.